Semiconductor device and method for manufacturing the same

ABSTRACT

Objects are to provide a semiconductor device for high power application in which a novel semiconductor material having high productivity is used and to provide a semiconductor device having a novel structure in which a novel semiconductor material is used. The present invention is a vertical transistor and a vertical diode each of which has a stacked body of an oxide semiconductor in which a first oxide semiconductor film having crystallinity and a second oxide semiconductor film having crystallinity are stacked. An impurity serving as an electron donor (donor) which is contained in the stacked body of an oxide semiconductor is removed in a step of crystal growth; therefore, the stacked body of an oxide semiconductor is highly purified and is an intrinsic semiconductor or a substantially intrinsic semiconductor whose carrier density is low. The stacked body of an oxide semiconductor has a wider band gap than a silicon semiconductor.

TECHNICAL FIELD

The present invention relates to a semiconductor device which has asemiconductor element using an oxide semiconductor and a manufacturingmethod of the semiconductor device.

BACKGROUND ART

A technique of forming a thin film transistor (TFT) by using a thinsemiconductor film formed at a relatively low temperature over asubstrate having an insulating surface has attracted attention. A thinfilm transistor is used for a display device typified by a liquidcrystal television. A silicon-based semiconductor material is known as amaterial for a semiconductor thin film applicable to a thin filmtransistor. Other than a silicon-based semiconductor material, an oxidesemiconductor has attracted attention.

As a material for the oxide semiconductor, zinc oxide and a materialcontaining zinc oxide as its component are known. Further, a thin filmtransistor formed using an amorphous oxide (an oxide semiconductor)having an electron carrier density lower than 10¹⁸ cm⁻³ is disclosed(Patent Documents 1 to 3).

REFERENCES Patent Documents

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165527-   [Patent Document 2] Japanese Published Patent Application No.    2006-165528-   [Patent Document 3] Japanese Published Patent Application No.    2006-165529

DISCLOSURE OF INVENTION

In the case of a transistor used in a semiconductor device for highpower application, characteristics such as high withstand voltage, highconversion efficiency, and high-speed switching are needed. Silicon isused for a semiconductor material of such a semiconductor device;however, from the above-described point of view, a novel semiconductormaterial which can further improve the characteristics is required.

As an example of a semiconductor material which can improve the abovecharacteristics, silicon carbide can be given. Since silicon carbide hasa small interatomic distance of approximately 0.18 nm in a Si—C bond, ahigh bond energy, and a large band gap which is about three times thatof silicon, it is known that silicon carbide is advantageous inincreasing the withstand voltage of a semiconductor device, reducing aloss of electric power, and the like.

However, it is difficult to melt silicon carbide because of itscharacteristics; therefore, silicon carbide cannot be manufactured by amethod having high productivity such as a Czochralski (CZ) method or thelike which is used for manufacturing a silicon wafer. Moreover, there isalso a problem in that silicon carbide has defects called micropipes.Because of these problems, commercialization of a semiconductor deviceusing silicon carbide is delayed.

In view of the foregoing problems, an object of one embodiment of thedisclosed invention is to provide a semiconductor device for high powerapplication in which a novel semiconductor material having highproductivity is used. Another object of one embodiment of the disclosedinvention is to provide a semiconductor device having a novel structurein which a novel semiconductor material is used.

In one embodiment of the present invention, a first oxide semiconductorfilm is formed over a first electrode which is formed over a substrate;then, crystal growth is caused from a surface to the inside portion ofthe first oxide semiconductor film by performing heat treatment at atemperature higher than or equal to 450° C. and lower than or equal to850° C., preferably higher than or equal to 550° C. and lower than orequal to 750° C. so that a first oxide semiconductor film havingcrystallinity which is in contact with the first electrode is formed;and a second oxide semiconductor film having crystallinity is stackedover the first oxide semiconductor film having crystallinity. Note thateach of the first oxide semiconductor films having crystallinity and thesecond oxide semiconductor film having crystallinity has aflat-plate-shaped polycrystalline region having uniform crystalalignment on the surface thereof. The flat-plate-shaped polycrystallineregion is c-axis aligned in a direction perpendicular to the surface ofthe first oxide semiconductor film having crystallinity and the secondoxide semiconductor film having crystallinity. Note that elements whichare adjacent to each other in the a-b plane are of the same kind. Thec-axis direction of each of the first oxide semiconductor film havingcrystallinity and the second oxide semiconductor film corresponds to thedirection perpendicular to the surface.

The second oxide semiconductor film having crystallinity can be formedin the following manner: a second oxide semiconductor film is formedover the first oxide semiconductor film having crystallinity; and then,heat treatment at a temperature higher than or equal to 450° C. andlower than or equal to 850° C., preferably higher than or equal to 550°C. and lower than or equal to 750° C. is performed so that crystalgrowth is caused toward the surface of the second oxide semiconductorfilm which is in an upper level than the surface of the first oxidesemiconductor film having crystallinity. That is, the first oxidesemiconductor film having crystallinity has a function of a seed crystalfor the second oxide semiconductor film.

Further, the second oxide semiconductor film having crystallinity isdeposited over the first oxide semiconductor film having crystallinitywhile heating is performed at a temperature higher than or equal to 200°C. and lower than or equal to 550° C. Typically, deposition is performedby a sputtering method, so that epitaxial growth or axial growth fromthe surface of the first oxide semiconductor film having crystallinityis caused and the second oxide semiconductor film having crystallinityis formed. That is, the first oxide semiconductor film havingcrystallinity has a function of a seed crystal for the second oxidesemiconductor film.

Since the crystal growth is caused with the use of the first oxidesemiconductor film having crystallinity as a seed crystal, the secondoxide semiconductor film having crystallinity has substantially the samecrystal alignment as the first oxide semiconductor film havingcrystallinity.

After that, the first and second oxide semiconductor films are etched tohave an island shape; a second electrode is formed over the second oxidesemiconductor film; and a gate insulating film and a third electrodefunctioning as a gate electrode are formed, whereby a verticaltransistor, a vertical diode, or the like can be manufactured as asemiconductor element. Note that the first electrode functions as one ofa source electrode and a drain electrode and the second electrodefunctions as the other of the source electrode and the drain electrode.

The heat treatment (first heat treatment) for forming the first oxidesemiconductor film having crystallinity and the heat treatment (secondheat treatment) for forming the second oxide semiconductor film havingcrystallinity are preferably performed in an atmosphere containingalmost no hydrogen and moisture (e.g., a nitrogen atmosphere, an oxygenatmosphere, or a dry-air atmosphere). By the first heat treatment andthe second heat treatment, dehydration or dehydrogenation by which H,OH, H₂O, or the like is eliminated from the first oxide semiconductorfilm is performed, whereby the first oxide semiconductor film havingcrystallinity and the second oxide semiconductor film havingcrystallinity can be highly purified. In addition, the first heattreatment and the second heat treatment can be performed in thefollowing manner: the temperature is increased in an inert gasatmosphere and then the atmosphere is switched to an atmospherecontaining oxygen. In the case where the heat treatment is performed inthe oxygen atmosphere, the oxide semiconductor film is oxidized, wherebyan oxygen deficiency can be repaired. Even when measurement usingthermal desorption spectroscopy (also referred to as TDS) at up to 450°C. is performed on the first oxide semiconductor film havingcrystallinity subjected to the first heat treatment, at least a peak ataround 300° C. of two peaks of water is not detected.

Note that in the case where the first oxide semiconductor film havingcrystallinity and the second oxide semiconductor film havingcrystallinity contain In, in their flat-plate-shaped polycrystallineregion, mobility is increased because electron clouds of In overlap witheach other to be connected to each other. Therefore, high field-effectmobility can be realized in a transistor which includes an oxidesemiconductor film having a polycrystalline region in a channel.

There is no particular limitation on a material of the first oxidesemiconductor film having crystallinity and that of the second oxidesemiconductor film having crystallinity and different materials may beused or materials that contain the same components may be used as longas a polycrystalline region which is c-axis aligned in a directionperpendicular to the surface can be obtained. Note that when differentmaterials are used, heteroepitaxial growth is caused and aheterospitaxial structure is obtained.

When an oxide semiconductor material used for forming the first oxidesemiconductor film having crystallinity and that used for forming thesecond oxide semiconductor film having crystallinity contain the samemain components, a boundary between the first oxide semiconductor filmhaving crystallinity and second oxide semiconductor film havingcrystallinity may become unclear and a substantially single-layerstructure may be obtained. Note that when materials that contain thesame components are used, homoepitaxial growth is caused and ahomoepitaxial structure is obtained.

Note that since a polycrystalline region having uniform crystalalignment which is formed at the surface of the first oxidesemiconductor film having crystallinity grows in a depth direction fromthe surface, the first oxide semiconductor film can be formed withoutbeing affected by a base member of the first oxide semiconductor filmhaving crystallinity in the case where the first oxide semiconductorfilm is amorphous just after being deposited.

A vertical transistor and a vertical diode each of which is oneembodiment of the present invention have a stacked body of an oxidesemiconductor in which the first oxide semiconductor film havingcrystallinity and the second oxide semiconductor film havingcrystallinity are stacked. An impurity serving as an electron donor(donor) which is contained in the stacked body of an oxide semiconductoris removed in a step of the crystal growth; therefore, the stacked bodyof an oxide semiconductor is highly purified and is formed using anintrinsic semiconductor or a substantially intrinsic semiconductor whosecarrier density is low. In addition, the stacked body of an oxidesemiconductor has a wider band gap than a silicon semiconductor.

In the highly purified stacked body of an oxide semiconductor, hydrogenconcentration is lower than or equal to 1×10¹⁸ cm⁻³, preferably lowerthan or equal to 1×10¹⁶ cm⁻³, more preferably substantially 0; carrierdensity is lower than 1×10¹² cm⁻³, preferably lower than 1.45×10¹⁰ cm⁻³,which is lower than the lower limit of measurement; and a band gap isgreater than or equal to 2 eV, preferably greater than or equal to 2.5eV, more preferably, greater than or equal to 3 eV.

By using such a highly purified stacked body of an oxide semiconductorin a channel formation region of a transistor, a channel can be formednot only at a surface of the stacked body of an oxide semiconductorwhich is in contact with a gate insulating film but also the insideportion of the stacked body of an oxide semiconductor, i.e., a channelcan be formed in the whole stacked body of an oxide semiconductor; thus,a large amount of current can flow in the transistor in an on state.When the transistor is in an off state, a depletion layer spreads to adeeper region inside the stacked body of an oxide semiconductor, wherebyoff-state current that is current flowing in a transistor in an offstate can be reduced. Further, withstand voltage becomes higher andhot-carrier degradation is suppressed so that a semiconductor device forhigh power application to which a high voltage is applied can bemanufactured.

By using such a highly purified stacked body of an oxide semiconductorin a diode, the diode has high rectification property.

Note that a transistor according to one embodiment of the presentinvention includes an insulated-gate field-effect transistor (IGFET) anda power MOSFET in its category.

According to one embodiment of the present invention, by using an oxidesemiconductor film in which hydrogen concentration is reduced and thepurity is increased and which has a polycrystalline region, a transistorand a diode can operate favorably. In a transistor, particularly,withstand voltage can be higher, a short channel effect can be reduced,and a high on-off ratio can be obtained. Therefore, a semiconductordevice for high power application can be manufactured by using thistransistor.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are a top view and a cross-sectional view illustrating atransistor;

FIG. 2 is a longitudinal cross-sectional view of a vertical transistorusing an oxide semiconductor;

FIGS. 3A and 3B are energy band diagrams (schematic diagrams) of a crosssection taken along line A-A′ in FIG. 2;

FIG. 4 is a diagram illustrating a relation between a vacuum level and awork function (φ_(M)) of metal and a relation between the vacuum leveland an electron affinity (χ) of an oxide semiconductor;

FIG. 5 is an energy band diagram (a schematic diagram) of a crosssection taken along line B-B′ in FIG. 2;

FIG. 6A illustrates a state where a positive potential (+V_(G)) isapplied to a gate

(GE1), and FIG. 6B illustrates a state where a negative potential(−V_(G)) is applied to the gate (GE1);

FIGS. 7A and 7B are a top view and a cross-sectional view illustrating atransistor;

FIGS. 8A and 8B are cross-sectional views each illustrating a diode;

FIGS. 9A to 9E are cross-sectional views illustrating a method formanufacturing a transistor;

FIGS. 10A to 10C are cross-sectional views illustrating the method formanufacturing the transistor;

FIGS. 11A and 11B are cross-sectional views illustrating the method formanufacturing the transistor;

FIGS. 12A to 12C are cross-sectional views illustrating the method formanufacturing the transistor;

FIGS. 13A to 13C are cross-sectional views illustrating a method formanufacturing a transistor;

FIGS. 14A and 14B are cross-sectional views illustrating a method formanufacturing a transistor;

FIGS. 15A to 15C are cross-sectional views illustrating a method formanufacturing a transistor;

FIGS. 16A to 16C are cross-sectional views illustrating a method formanufacturing a transistor;

FIGS. 17A and 17B are cross-sectional views illustrating a method formanufacturing a transistor;

FIG. 18 is a diagram illustrating an example of a photovoltaic system;

FIGS. 19A and 19B are a TEM photograph and a schematic diagram of across section of an oxide semiconductor film; and

FIGS. 20A and 20B are a TEM photograph and a schematic diagram of across section of an oxide semiconductor film.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below withreference to the accompanying drawings. However, the present inventionis not limited to the description below, and it is easily understood bythose skilled in the art that modes and details disclosed herein can bemodified in various ways without departing from the spirit and scope ofthe present invention. Therefore, the present invention should not beinterpreted as being limited to description in the embodiments below.Note that in structures of the present invention described hereinafter,like portions or portions having similar functions are denoted by thesame reference numerals in different drawings, and description thereofis not repeated.

Note that in each drawing described in this specification, the size ofeach component or the thickness of each layer or an area is exaggeratedin some cases for clarification. Therefore, embodiments of the presentinvention are not limited to such scales.

Note that the numeral terms such as “first”, “second”, and “third” inthis specification are used in order to avoid confusion betweencomponents and do not set a limitation on number. Therefore, forexample, the term “first” can be replaced with the term “second”,“third”, or the like as appropriate.

Note that “voltage” indicates a difference between potentials of twopoints, and “potential” indicates electrostatic energy (electricalpotential energy) of a unit charge at a given point in an electrostaticfield. Note that in general, a difference between potential of one pointand a reference potential is merely called potential or voltage, andpotential and voltage are used as synonymous words in many cases. Thus,in this specification, potential may be rephrased as voltage and voltagemay be rephrased as potential unless otherwise specified.

Embodiment 1

In this embodiment, a structure of a transistor, which is an embodimentof a semiconductor element, will be described with reference to FIGS. 1Aand 1B.

FIG. 1A is a top view of a transistor 145, and FIG. 1B corresponds to across-sectional view taken along dashed line A-B in FIG. 1A.

As illustrated in FIG. 1B, a first electrode 105, an oxide semiconductorfilm 107 which has a polycrystalline region, and a second electrode 109are stacked over an insulating film 103 formed over a substrate 101.Note that the second electrode 109 is stacked over and in contact withthe entire top surface of the oxide semiconductor film 107. A gateinsulating film 111 is provided so as to cover the first electrode 105,the oxide semiconductor film 107, and the second electrode 109. Over thegate insulating film 111, a third electrode 113 facing at least a sidesurface of the oxide semiconductor film is provided. An insulating film117 functioning as an interlayer insulating film is provided over thegate insulating film 111 and the third electrode 113. Openings areformed in the insulating film 117, and a wiring 131 (see FIG. 1A)connected through the opening to the first electrode 105, a wiring 129connected through the opening to the second electrode 109, and a wiring125 connected through the opening to the third electrode 113 are formed.Note that in this specification a top surface of a film means a surfaceout of a pair of surfaces parallel to the substrate 101 which is farfrom the substrate 101.

The first electrode 105 functions as one of a source electrode and adrain electrode of the transistor 145. The second electrode 109functions as the other of the source electrode and the drain electrodeof the transistor 145. The third electrode 113 functions as a gateelectrode of the transistor 145.

In this embodiment, the oxide semiconductor film 107 has crystallinityand has a flat-plate-shaped polycrystalline region in which crystalalignment is uniform at a surface thereof. That is, the polycrystallineregion of the oxide semiconductor film 107 has the a-b plane which isparallel to the surface and is c-axis aligned in a directionperpendicular to the surface. In other words, the c-axis direction ofthe oxide semiconductor film 107 corresponds to the directionperpendicular to the surface. Note that elements which are adjacent toeach other in the a-b plane are of the same kind. Note that theflat-plate-shaped polycrystalline region is a region which has aplurality of single crystal regions in each of which c-axis is alignedin the direction perpendicular to the surface.

In the polycrystalline region, electron clouds of In overlap with eachother to be connected to each other, whereby electrical conductivity σis improved. Therefore, high field-effect mobility can be realized in atransistor which includes an oxide semiconductor film having apolycrystalline region.

The oxide semiconductor film 107 is formed using metal oxide and any oneof the following metal oxide films can be used: a four-component metaloxide film such as an In—Sn—Ga—Zn—O film; a three-component metal oxidefilm such as an In—Ga—Zn—O film, an In—Sn—Zn—O film, an In—Al—Zn—O film,a Sn—Ga—Zn—O film, an Al—Ga—Zn—O film, and a Sn—Al—Zn—O film; atwo-component metal oxide film such as an In—Zn—O film, a Sn—Zn—O film,an Al—Zn—O film, a Zn—Mg—O film, a Sn—Mg—O film, and an In—Mg—O film; anIn—O film; a Sn—O film; and a Zn—O film.

For the oxide semiconductor film 107, a material expressed byInMO₃(ZnO)_(m) (m>0) can be used. Here, M represents one or more metalelements selected from Ga, Al, Mn, and Co. For example, M can be Ga, Gaand Al, Ga and Mn, Ga and Co, or the like.

Note also that the oxide semiconductor film 107 may be formed using anoxide semiconductor material represented by In-A-B—O. Here, A representsone or more kinds of elements selected from Group 13 elements such asgallium (Ga) and aluminum (Al), Group 14 elements such as silicon (Si)and germanium (Ge), and the like. B represents one or more kinds ofelements selected from Group 12 elements such as zinc (Zn). It is to benoted that values of an In content, an A content, and a B content arearbitrary. The value of the A content may be 0. On the other hand, thevalue of the In content and that of the B content are not 0. In otherwords, the above expression may represent In—Ga—Zn—O, In—Zn—O, and thelike.

In the case where the oxide semiconductor film 107 has a crystalstructure represented by In₂Ga₂ZnO₇, or InGaZnO₄, it can be understoodthat the oxide semiconductor film 107 contains any of In, Ga, and Zn andhas a stacked-layer structure of layers parallel to the a-axis andb-axis. Since electrical conductivity of the crystals of InGaZnO₄ orIn₂Ga₂ZnO₇ are controlled mainly by In, electrical characteristics of alayer containing In which are related to a direction parallel to thea-axis and b-axis are preferable. In the crystals of InGaZnO₄ orIn₂Ga₂ZnO₇, electron clouds of In overlap with each other to beconnected to each other so that a carrier path is formed.

In other words, crystallization is caused more easily in the a-b planedirection than in the c-axis direction. Further, in theflat-plate-shaped polycrystalline region, the a-b planes of the singlecrystal regions become parallel to the surface. In addition, a freespace is located above the surface of the oxide semiconductor film 107,in which crystals do not grow upward. These can be inferred from thefact that when an In—Ga—Zn—O film as the oxide semiconductor film 107was measured with thermal desorption spectroscopy (TDS) while thetemperature was increased to 450° C., peaks of In and Ga were notdetected but a peak of zinc was detected in a vacuum heating condition,particularly at around 300° C. Note that the TDS measurement wasperformed in a vacuum and it was observed that elimination of zinc wasdetected from around 200° C.

A conventional oxide semiconductor is generally an n-type semiconductorand current tends to flow between source and drain electrodes even whena gate voltage is 0 V in a transistor using an oxide semiconductor; thatis, the transistor tends to be normally on. In the case where thetransistor is normally on, it is difficult to control the circuit evenwhen the field-effect mobility is high. Note that it is known that somehydrogen is a donor in an oxide semiconductor and is one factor causingan oxide semiconductor to be an n-type semiconductor. It is also knownthat some oxygen deficiency is a donor and is another factor causing anoxide semiconductor to be an n-type semiconductor.

Therefore, in order to make the oxide semiconductor film be an i-typeoxide semiconductor film, the oxide semiconductor film is highlypurified by removing hydrogen that is an n-type impurity from the oxidesemiconductor film so as to contain an impurity that is not a maincomponent of the oxide semiconductor film as little as possible and ismade intrinsic (i-type) or substantially intrinsic by removing oxygendeficiency. In other words, a feature of one embodiment of the presentinvention is that a highly purified i-type (intrinsic) semiconductor, ora semiconductor close thereto, is obtained not by adding an impurity butby removing an impurity such as hydrogen or water or oxygen deficiencyas much as possible. By highly purifying the oxide semiconductor film,the threshold voltage of the transistor can be positive and a so-callednormally-off switching element can be obtained.

The hydrogen concentration in the oxide semiconductor film 107 here islower than or equal to 1×10¹⁸ cm⁻³, preferably lower than or equal to1×10¹⁶ cm⁻³, more preferably substantially 0. The carrier density of theoxide semiconductor film 107 is lower than 1×10¹² cm⁻³, preferably lowerthan 1.45×10¹⁰ cm⁻³, which is lower than the lower limit of measurement.That is, the carrier density of the oxide semiconductor film is as closeto zero as possible. A band gap is greater than or equal to 2 eV,preferably greater than or equal to 2.5 eV, more preferably greater thanor equal to 3 eV. Note that the hydrogen concentration in the oxidesemiconductor film can be measured by secondary ion mass spectrometry(SIMS). The carrier density can be measured by the Hall effectmeasurement. Lower carrier density can be calculated with the use ofmeasurement results of capacitance-voltage (CV) measurement and Formula1.

$\begin{matrix}{N_{d} = {{- \left( \frac{2}{e\; ɛ_{0}ɛ} \right)}/\frac{{d\left( {1/C} \right)}^{2}}{dV}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

The thickness of the oxide semiconductor film 107 is greater than orequal to 1 μm, preferably greater than or equal to 3 μm, more preferablygreater than or equal to 10 μm, in which case a semiconductor device forhigh power application can be manufactured.

In this embodiment, the third electrode 113 functioning as the gateelectrode has a ring shape. When the third electrode 113 functioning asthe gate electrode has a ring shape, the channel width of the transistorcan be increased. In the transistor of this embodiment, the channellength L means the thickness of the oxide semiconductor film and thechannel width W is a length of an end portion of the oxide semiconductorfilm that is in contact with the first electrode or the secondelectrode. Note that in this embodiment, the length of the end portionof the oxide semiconductor film that is in contact with one of the firstelectrode and the second electrode which has a larger area than theother is W. In this embodiment, since the shape of the top surface ofthe oxide semiconductor film of the transistor is a rectangle with aside W₁ and a side W₂, the channel width W is the sum of 2W₁ and 2W₂.Note that in the case where the shape of the top surface of the oxidesemiconductor film of the transistor is circular, the channel width W is2πr where r is a radius of the circle.

The transistor of this embodiment has large on-state current because theoxide semiconductor film included therein has the polycrystalline regionso that the whole oxide semiconductor film functions as a channel in anon state and more carriers transfer. Further, in the transistor of thisembodiment, the intrinsic carrier density is extremely low and thus themaximum width of a depletion layer is extremely large, and the depletionlayer spreads inside the oxide semiconductor film in an off state andoff-state current is reduced. In other words, a high on-off ratio can beobtained in the transistor.

Note that a transistor is an element having at least three terminals: agate, a drain, and a source. The transistor has a channel formationregion between a drain region and a source region, and current can flowthrough the drain region, the channel formation region, and the sourceregion. Here, since the source and the drain of the transistor areinterchangeable depending on a structure, operating conditions, and thelike of the transistor, it is difficult to define which is a source or adrain. Thus, a region which serves as a source and a drain is notreferred to as a source or a drain in some cases. In such a case, one ofthe source and the drain may be referred to as a first terminal and theother thereof may be referred to as a second terminal, for example.Alternatively, one of the source and the drain may be referred to as afirst electrode and the other thereof may be referred to as a secondelectrode. Alternatively, one of the source and the drain may bereferred to as a first region and the other of the source and the drainmay be referred to as a second region.

It is necessary that the substrate 101 have at least enough heatresistance to withstand heat treatment to be performed later. As thesubstrate 101, a glass substrate of barium borosilicate glass,aluminoborosilicate glass, or the like can be used.

As the glass substrate, in the case where the temperature of the heattreatment to be performed later is high, the one whose strain point ishigher than or equal to 730° C. is preferably used. As the glasssubstrate, a glass material such as aluminosilicate glass,aluminoborosilicate glass, or barium borosilicate glass is used, forexample. Note that a glass substrate containing BaO and B₂O₃ so that theamount of BaO is larger than that of B₂O₃ is preferably used.

Note that, instead of the glass substrate described above, a substrateformed using an insulator, such as a ceramic substrate, a quartzsubstrate, or a sapphire substrate, may be used. Alternatively,crystallized glass or the like may be used.

The insulating film 103 is formed using an oxide insulating film such asa silicon oxide film or a silicon oxynitride film; or a nitrideinsulating film such as a silicon nitride film, a silicon nitride oxidefilm, an aluminum nitride film, or an aluminum nitride oxide film. Inaddition, the insulating film 103 may have a stacked-layer structure,for example, a stacked-layer structure in which one or more of thenitride insulating films and one or more of the oxide insulating filmsare stacked in that order over the substrate 101. The thickness of theinsulating film 103 is preferably greater than or equal to 100 nm andless than or equal to 2 μm.

The first electrode 105 and the second electrode 109 are formed using ametal element selected from aluminum, chromium, copper, tantalum,titanium, molybdenum, tungsten, and yttrium; an alloy containing any ofthese metal elements as a component; an alloy containing these metalelements in combination; or the like. Alternatively, one or more metalelements selected from manganese, magnesium, zirconium, beryllium, andthorium can be used. In addition, the first electrode 105 can have asingle-layer structure or a stacked-layer structure having two or morelayers. For example, a single-layer structure of an aluminum filmcontaining silicon, a two-layer structure in which a titanium film isstacked over an aluminum film, a two-layer structure in which a titaniumfilm is stacked over a tungsten film, a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked in thisorder, and the like can be given. Alternatively, a film, an alloy film,or a nitride film which contains aluminum and one or more elementsselected from titanium, tantalum, tungsten, molybdenum, chromium,neodymium, and scandium may be used.

The first electrode 105 and the second electrode 109 can be formed usinga light-transmitting conductive material such as indium tin oxide,indium oxide containing tungsten oxide, indium zinc oxide containingtungsten oxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium zinc oxide, or indium tin oxide towhich silicon oxide is added. It is also possible to have astacked-layer structure having a layer containing the abovelight-transmitting conductive material and a layer containing the abovemetal element.

The gate insulating film 111 can be formed to have a single-layerstructure or a stacked-layer structure having a silicon oxide film, asilicon nitride film, a silicon oxynitride film, a silicon nitride oxidefilm, and/or an aluminum oxide film. A portion of the gate insulatingfilm 111 which is in contact with the oxide semiconductor film 107preferably contains oxygen, and it is particularly preferable that theportion of the gate insulating film 111 be formed using a silicon oxidefilm. By using a silicon oxide film, oxygen can be supplied to the oxidesemiconductor film 107 and favorable characteristics can be obtained.

The gate insulating film 111 is formed using a high-k material such ashafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen isadded (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so that gateleakage current can be reduced. Further, a stacked-layer structure inwhich a high-k material and one or more of a silicon oxide film, asilicon nitride film, a silicon oxynitride film, a silicon nitride oxidefilm, and an aluminum oxide film are stacked can be used. The thicknessof the gate insulating film 111 is preferably greater than or equal to50 nm and less than or equal to 500 nm. The large thickness of the gateinsulating film 111 makes it possible to reduce the gate leakagecurrent.

The third electrode 113 functioning as a gate electrode is formed usinga metal element selected from aluminum, chromium, copper, tantalum,titanium, molybdenum, and tungsten; an alloy containing any of thesemetal elements as a component; an alloy film containing these metalelements in combination; or the like. Further, one or more metalelements selected from manganese, magnesium, zirconium, and berylliummay be used. In addition, the third electrode 113 can have asingle-layer structure or a stacked-layer structure having two or morelayers. For example, a single-layer structure of an aluminum filmcontaining silicon, a two-layer structure in which a titanium film isstacked over an aluminum film, a two-layer structure in which a titaniumfilm is stacked over a titanium nitride film, a two-layer structure inwhich a tungsten film is stacked over a titanium nitride film, atwo-layer structure in which a tungsten film is stacked over a tantalumnitride film, a three-layer structure in which a titanium film, analuminum film, and a titanium film are stacked in this order, and thelike can be given. Alternatively, a film, an alloy film, or a nitridefilm which contains aluminum and one or a plurality of elements selectedfrom titanium, tantalum, tungsten, molybdenum, chromium, neodymium, andscandium may be used.

The third electrode 113 functioning as the gate electrode can be formedusing a light-transmitting conductive material such as indium tin oxide,indium oxide containing tungsten oxide, indium zinc oxide containingtungsten oxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium zinc oxide, or indium tin oxide towhich silicon oxide is added. It is also possible to have astacked-layer structure formed using the above light-transmittingconductive material and the above metal element.

Next, operation of the transistor including the oxide semiconductor film107 will be described with reference to energy band diagrams.

FIG. 2 is a longitudinal cross-sectional view of a vertical transistorin which an oxide semiconductor is used. An oxide semiconductor film(OS) is provided over a drain electrode (D), and a source electrode (S)is provided over the oxide semiconductor film (OS). A gate insulatingfilm (GI) is provided over the drain electrode, the oxide semiconductorfilm, and the source electrode, and a gate electrode (GE1) is providedthereover.

FIGS. 3A and 3B are energy band diagrams (schematic diagrams) of a crosssection taken along line A-A′ in FIG. 2. FIG. 3A illustrates the casewhere a voltage applied to the source is equal to a voltage applied tothe drain (V_(D)=0 V), and FIG. 3B illustrates the case where a positivepotential with respect to the source is applied to the drain (V_(D)>0)and a positive voltage is applied to the gate electrode (V_(G)>0).

FIG. 5 is an energy band diagram (a schematic diagram) of a crosssection taken along line B-B′ in FIG. 2 in the case where the gatevoltage is 0 V. FIG. 6A illustrates a state where a positive potential(+V_(G)) is applied to the gate (GE1), in other words, a case where thetransistor is in an on-state where carriers (electrons) flow between thesource and the drain. FIG. 6B illustrates a state where a negativepotential (−V_(G)) is applied to the gate (GE1), in other words, a casewhere the transistor is in an off-state.

FIG. 4 illustrates a relation between a vacuum level and a work function(φ_(M)) of metal and a relation between the vacuum level and an electronaffinity (χ) of an oxide semiconductor.

Metal is degenerated and the Fermi level is located in the conductionband. On the other hand, a conventional oxide semiconductor is typicallyan n-type semiconductor, in which case the Fermi level (E_(F)) is awayfrom the intrinsic Fermi level (E_(i)) located in the middle of a bandgap and is located closer to the conduction band. Note that it is knownthat some hydrogen is a donor in an oxide semiconductor and is onefactor causing an oxide semiconductor to be an n-type semiconductor.

On the other hand, an oxide semiconductor according to one embodiment ofthe present invention is an intrinsic (i-type) semiconductor or asubstantially intrinsic semiconductor which is obtained by removinghydrogen that is an n-type impurity from an oxide semiconductor andhighly purifying the oxide semiconductor so that impurities that are notmain components of the oxide semiconductor are prevented from beingcontained therein as much as possible. In other words, a feature of oneembodiment of the present invention is that a highly purified i-typesemiconductor, or a semiconductor close thereto, is obtained not byadding an impurity but by removing an impurity such as hydrogen or wateras much as possible. This enables the Fermi level (E_(F)) to be at thesame level as the intrinsic Fermi level (E_(i)).

In the case where the band gap (E_(g)) of an oxide semiconductor is 3.15eV, the electron affinity (χ) is said to be 4.3 eV. The work function oftitanium (Ti) contained in the source electrode and the drain electrodeis substantially equal to the electron affinity (χ) of the oxidesemiconductor. In that case, a Schottky barrier for electrons is notformed at an interface between the metal and the oxide semiconductor.

That is, in the case where the work function of metal (φ_(M)) and theelectron affinity (χ) of the oxide semiconductor are equal to each otherand the metal and the oxide semiconductor are in contact with eachother, an energy band diagram (a schematic diagram) as illustrated inFIG. 3A is obtained.

In FIG. 3B, a black circle (•) represents an electron, and when apositive potential is applied to the drain, the electron is injectedinto the oxide semiconductor over the barrier (h) and flows toward thedrain. In that case, the height of the barrier (h) changes depending onthe gate voltage and the drain voltage; in the case where a positivedrain voltage is applied, the height of the barrier (h) is smaller thanthe height of the barrier in FIG. 3A where no voltage is applied, i.e.,½ of the band gap (E_(g)).

The thickness of the oxide semiconductor film is greater than or equalto 1 μm, preferably greater than or equal to 3 μm, more preferablygreater than or equal to 10 μm, and the carrier density is low. Thus, inthe state where a positive potential (+V_(G)) is applied to the gate(GE1), as illustrated in FIG. 6A, the degree of curve of the band at asurface of the oxide semiconductor layer is small, the lower end of theconduction band approaches the Fermi level, and the entire oxidesemiconductor film is stable in terms of energy. Therefore, electronsflow more easily not only in the vicinity of the gate insulating filmbut also in the entire region of the oxide semiconductor; as a result, achannel is formed in the entire region of the oxide semiconductor and alarger amount of current can flow. On the other hand, off-state current,i.e., current which flows in the state where a negative potential(−V_(G)) is applied to the gate (GE1), is made to flow by generation andrecombination of electrons and holes through direct recombination orindirect recombination; however, since an oxide semiconductor has a wideband gap and a large amount of thermal energy is needed for electronicexcitation, direct recombination and indirect recombination are lesslikely to occur. Thus, in the state where a negative potential (−V_(G))is applied to the gate (GE1), since the number of holes that areminority carriers is substantially zero, direct recombination andindirect recombination are less likely to occur and the amount ofcurrent is extremely small; as a result, the value of current in achannel per unit area is lower than or equal to 100 aA/μm, preferablylower than or equal to 10 aA/μm, more preferably lower than or equal to1 aA/μm, which is close to zero.

Next, the intrinsic carrier density of the oxide semiconductor will bedescribed.

The intrinsic carrier density n, contained in a semiconductor iscalculated by approximation of Fermi-Dirac distribution in accordancewith Fermi-Dirac statics by the Boltzmann distribution formula (seeFormula 2).

$\begin{matrix}{n_{i} = {\sqrt{N_{C}N_{V}}{\exp \left( {- \frac{E_{g}}{2\; {kT}}} \right)}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$

The intrinsic carrier density n_(i) obtained by the approximateexpression is a relational expression of effective density of states ina conduction band N_(C), effective density of states in a valence bandN_(V), and a band gap E_(g). From Formula 2, the intrinsic carrierdensity n_(i) of silicon is 1.45×10¹⁰ cm⁻³, and the intrinsic carrierdensity n_(i) of an oxide semiconductor (an In—Ga—Zn—O film here) is1.2×10⁻⁷ cm⁻³; that is, the carrier density of silicon is 10¹⁷ timesthat of an oxide semiconductor. In other words, it is found that theintrinsic carrier density of an oxide semiconductor is extremely low ascompared with that of silicon.

Next, the width of a depletion layer and the Debye length in the casewhere a negative potential (−V_(G)) is applied to the gate (GE1) will bedescribed below.

When voltage is applied to a MOS transistor formed using a semiconductorhaving a donor density N_(d), an insulator, and a metal, the maximumwidth of a depletion layer T_(D MAX) formed in the semiconductor can becalculated by Formula 3.

$\begin{matrix}{T_{D\mspace{11mu} {MAX}} = \sqrt{\frac{2ɛ_{S}{ɛ_{0}\left( {2\varphi_{F}} \right)}}{{qN}_{d}}}} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack\end{matrix}$

The maximum width of a depletion layer can be expressed as a functionalof the donor density and a Fermi potential, and the Fermi potentialφ_(F) can be calculated by Formula 4.

$\begin{matrix}{\varphi_{F} = {\frac{kT}{q}\ln \frac{N_{d}}{n_{i}}}} & \left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack\end{matrix}$

The Debye length L_(D) of the MOS transistor can be calculated byFormula 5.

$\begin{matrix}{L_{D} = \sqrt{\frac{ɛ_{S}ɛ_{0}{kT}}{q^{2}N_{d}}}} & \left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Note that ε_(s), ε₀N_(d), q, k, and T respectively represent dielectricconstant of an oxide semiconductor, permittivity of a vacuum, donordensity, elementary electric charge, Boltzmann constant, andtemperature.

The maximum width of a depletion layer and the Debye length of a MOStransistor using silicon and the maximum width of a depletion layer andthe Debye length of a MOS transistor using an oxide semiconductor wascalculated, where n_(i) (intrinsic carrier density) of silicon, ε_(s)thereof, n_(i) of the oxide semiconductor, and ε_(s) thereof arerespectively set to 1.45×10¹⁰ cm⁻³, 11.9, 1.2×10⁻⁷ cm⁻³, and 10. Each ofthe transistors here used for the calculation has a horizontal MOStransistor structure in which a channel is formed in parallel to asurface of a substrate. Note that the maximum width of the depletionlayer here corresponds to the width of the depletion layer that spreadsperpendicular to the substrate. Note also that a depletion layer in avertical MOS transistor spreads in a manner similar to that of thedepletion layer in the horizontal MOS transistor.

When silicon is used, the donor density corresponds to density ofimpurities (P). When an oxide semiconductor is used, oxygen deficiencyand hydrogen contribute to formation of donors.

As the donor density is decreased, the maximum width of a depletionlayer calculated by Formula 4 is increased and the Debye lengthcalculated by Formula 5 is increased. It is also found that the maximumwidth of a depletion layer T_(D MAX) depends on the intrinsic carrierdensity n_(i) and that the depletion layer spreads more when the oxidesemiconductor having lower is used than when silicon is used. Further,as the donor density (N_(d)) is decreased, the Debye length is increasedand the depletion layer spreads in the whole oxide semiconductor.Typically, when the donor density is from 1×10¹² cm⁻³ to 1×10¹⁸ cm⁻³,the maximum width of a depletion layer in an oxide semiconductor andsilicon is on the order of submicrons or several tens of micrometers andthe Debye length in the oxide semiconductor and silicon is from severalnanometers to several micrometers. It is also found that, when the donordensity ranges from 1×10⁻⁵ cm⁻³ to 1×10¹ cm⁻³, the maximum width of adepletion layer in the oxide semiconductor is drastically increased fromseveral tens micrometers to several thousands of micrometers, the Debyelength is drastically increased from several micrometers to severalhundreds of micrometers, and the depletion layer spreads in the wholeoxide semiconductor.

From the above, since the oxide semiconductor has a wide band gap and alow intrinsic carrier density, the maximum width of a depletion layerand the Debye length are increased, and the depletion layer spreads inthe entire region of the oxide semiconductor in an off-state; as aresult, the off-state current can be reduced to a value as close to zeroas possible.

By highly purifying the oxide semiconductor to make an intrinsic(i-type) semiconductor or a substantially intrinsic semiconductor sothat impurities that are not main components of the oxide semiconductorare not contained as much as possible, the interface characteristicswith a gate insulating film become obvious. Therefore, the gateinsulating film is preferably formed using a material that can form afavorable interface with the oxide semiconductor. For example, it ispreferable to use a dense insulating film formed by a CVD method usinghigh-density plasma generated with a power supply frequency in the rangeof the VHF band to the microwave band, or an insulating film formed by asputtering method. Further, in order to obtain a favorable interfacebetween the gate insulating film and a gate electrode, on a surface ofthe gate insulating film, a dense insulating film formed by a CVD methodusing high-density plasma generated with a power supply frequency in therange of the VHF band to the microwave band may be formed.

By highly purifying an oxide semiconductor in this manner so thatimpurities that are not main components of the oxide semiconductor arenot contained as much as possible, a transistor with high on-statecurrent, a low off-state current, a high on/off ratio, and favorableoperation characteristics can be manufactured. Further, fluctuation inon-state current, field-effect mobility, and a threshold voltage due totemperature change is extremely small.

Here, the drain withstand voltage of a transistor using an oxidesemiconductor will be described.

When the electric field in the semiconductor reaches a certain thresholdvalue, impact ionization occurs, carriers accelerated by the highelectric field impact crystal lattices in a depletion layer, therebygenerating pairs of electrons and holes. When the electric field becomeseven higher, the pairs of electrons and holes generated by the impactionization are further accelerated by the electric field, and the impactionization is repeated, resulting in an avalanche breakdown in whichcurrent is increased exponentially. The impact ionization occurs becausecarriers (electrons and holes) have kinetic energy that is larger thanor equal to the band gap of the semiconductor. The impact ionizationcoefficient that shows probability of impact ionization has correlationwith the band gap. It is known that the impact ionization is unlikely tooccur as the band gap is increased.

Since the band gap of the oxide semiconductor is 3.15 eV, which islarger than the band gap of silicon, 1.12 eV, the avalanche breakdown isexpected to be unlikely to occur. Therefore, a transistor using theoxide semiconductor has a high drain withstand voltage, and anexponential sudden increase of on-state current is expected to beunlikely to occur when a high electric field is applied.

Next, hot-carrier degradation of a transistor using an oxidesemiconductor will be described.

The hot-carrier degradation means a phenomenon that electrons that areaccelerated to be rapid become a fixed charge by being injected in agate oxide film in the vicinity of a drain in a channel or form a traplevel at the interface between the gate insulating film and the oxidesemiconductor film. The factors of the hot-carrier degradation arechannel-hot-electron injection (CHE injection) anddrain-avalanche-hot-carrier injection (DAHC injection).

Since the band gap of silicon is narrow, electrons are likely to begenerated like an avalanche owing to an avalanche breakdown, andelectrons that are accelerated to be so rapid as to go over a barrier tothe gate insulating film are increased in number. However, the oxidesemiconductor described in this embodiment has a wide band gap;therefore, the avalanche breakdown is unlikely to occur and resistanceto the hot-carrier degradation is higher than that of silicon. Note thatalthough the band gap of silicon carbide which is one of materialshaving high withstand voltage and that of an oxide semiconductor aresubstantially equal to each other, electrons are less likely to beaccelerated in an oxide semiconductor because the mobility in an oxidesemiconductor is approximately one-hundredth as high as that of siliconcarbide. Further, a barrier between an oxide semiconductor and an oxidefilm that is a gate insulating film is larger than a barrier between anyof silicon carbide, gallium nitride, and silicon and an oxide film thatis a gate insulating film; therefore, in the case of an oxidesemiconductor, the number of electrons injected to the oxide film isextremely small, hot-carrier degradation is less likely to be causedthan in the case of silicon carbide, gallium nitride, or silicon, and itcan be said that drain withstand voltage is high. Thus,low-concentration impurity regions do not need to be provided between anoxide semiconductor functioning as a channel and source and drainelectrodes so that an extremely simple transistor structure can berealized and the number of manufacturing steps of which can be reduced.

From the above, a transistor using an oxide semiconductor has high drainwithstand voltage; specifically, such a transistor can have drainwithstand voltage greater than or equal to 100 V, preferably greaterthan or equal to 500 V, more preferably greater than or equal to 1 kV.

Comparison between a transistor using silicon carbide, which is atypical example of a transistor, and a transistor using an oxidesemiconductor will be described below. Here, 4H—SiC is used as thesilicon carbide.

An oxide semiconductor and 4H—SiC have some things in common. Oneexample is intrinsic carrier density. Using the Fermi-Dirac distributionat normal temperature, the intrinsic carrier density of the oxidesemiconductor is estimated to approximately 10⁻⁷ cm⁻³, which isextremely low like the carrier density of 4H—SiC, i.e., 6.7×10⁻¹¹ cm⁻³.

In addition, the energy band gap of the oxide semiconductor is 3.0 eV to3.5 eV and that of 4H—SiC is 3.26 eV, which means that both the oxidesemiconductor and the silicon carbide are wide-gap semiconductors.

However, the manufacturing temperature of transistors using an oxidesemiconductor and silicon carbide is largely different. Heat treatmentfor activation at 1500° C. to 2000° C. is needed in the case of usingsilicon carbide. In contrast, in the case of using an oxidesemiconductor, an oxide semiconductor having crystallinity can bemanufactured by heat treatment at a temperature higher than or equal to450° C. and lower than or equal to 850° C., preferably higher than orequal to 550° C. and lower than or equal to 750° C., which allows atransistor to be manufactured over a large-sized substrate. In addition,throughput can be increased.

A manufacturing process of a SiC-MOSFET includes a step of doping withan impurity that can be a donor or an acceptor (e.g., phosphorus orboron) and a high-temperature heat treatment step for activation. Here,it is to be noted that an oxide semiconductor has a relatively highelectron affinity. Accordingly, by selecting metal having an appropriatework function for an electrode, an ohmic contact can be formed betweenthe oxide semiconductor and the electrode without a step of doping withan impurity in a manufacturing process of a transistor. In this manner,simplification of the process can be realized because an n⁺ region iseasily formed in the contact portion.

Note that considerable research has been done on properties of oxidesemiconductors such as density of states (DOS) in the band gap; however,the research does not include the idea of sufficiently reducing the DOSitself. In this embodiment, a highly purified oxide semiconductor ismanufactured by removing water or hydrogen which might induce the DOS inthe energy gap from the oxide semiconductor. This is based on the ideaof sufficiently reducing the DOS itself. Thus, excellent industrialproducts can be manufactured.

Further, it is also possible to form a more highly purified (i-type)oxide semiconductor by supplying oxygen to a dangling bond of metalwhich is generated by lack of oxygen and reducing the DOS due to lack ofoxygen. For example, an oxide film containing an excessive amount ofoxygen is formed in close contact with a channel formation region andoxygen is supplied from the oxide film, whereby the DOS due to oxygendeficiency can be reduced.

It is said that a defect of the oxide semiconductor is caused by ashallow level of 0.1 eV to 0.2 eV below the conduction band due toexcessive hydrogen, a deep level due to lack of oxygen, or the like. Thetechnical idea that hydrogen is drastically reduced and oxygen issufficiently supplied in order to eliminate such a defect would beright.

An oxide semiconductor is generally considered as an n-typesemiconductor; however, in this embodiment, an i-type semiconductor isrealized by removing an impurity, particularly water or hydrogen. Inthis respect, it can be said that one embodiment of the presentinvention includes a novel technical idea because it is different froman i-type semiconductor such as silicon added with an impurity. Inaddition, density of a heavy element, e.g., an impurity such as iron ornickel, which is not contained in the oxide semiconductor is preferablyless than or equal to 1×10¹⁵ cm⁻³.

By making the oxide semiconductor be an i-type semiconductor, favorabletemperature characteristics of the transistor can be obtained;specifically, in terms of the current vs. voltage characteristics of thetransistor, on-state current, off-state current, field-effect mobility,an S value, and a threshold voltage are hardly fluctuated at atemperature ranging from −25° C. to 150° C., and the current vs. voltagecharacteristics are hardly degraded by the temperature.

In the transistor using an oxide semiconductor which is described inthis embodiment, mobility at a channel is a little lower than that in atransistor using silicon carbide; however, a current value and devicecharacteristics of the transistor can be improved by increasing thedrain voltage and the channel width (W).

A technical idea of this embodiment is that an impurity is not added toan oxide semiconductor and on the contrary the oxide semiconductoritself is highly purified by intentionally removing an impurity such aswater or hydrogen which undesirably exists therein. In other words, theoxide semiconductor is highly purified by removing water or hydrogenwhich forms a donor level, reducing oxygen deficiency, and sufficientlysupplying oxygen that is a main component of the oxide semiconductor.

In the oxide semiconductor just after being deposited, hydrogen atdensity of 1×10²⁰ cm⁻³ to 9×10²⁰ cm⁻³ is measured using secondary ionmass spectrometry (SIMS). The oxide semiconductor is highly purified andmade to be an i-type (intrinsic) semiconductor by intentionally removingwater or hydrogen which forms a donor level and further by adding oxygen(one of components of the oxide semiconductor), which is reduced at thesame time as removal of water or hydrogen, to the oxide semiconductor.

In this embodiment, the amount of water and hydrogen in the oxidesemiconductor is preferably as small as possible, and the number ofcarriers in the oxide semiconductor is preferably as small as possible.In other words, carrier density lower than 1×10¹² cm⁻³, or preferablylower than 1.45×10¹⁰ cm⁻³, i.e., lower than the lower limit ofmeasurement is desirable. Further, in the technical idea of thisembodiment, an ideal carrier density is 0 or close to 0. In particular,the oxide semiconductor can be highly purified in such a manner that theoxide semiconductor is subjected to heat treatment in an oxygenatmosphere, a nitrogen atmosphere, or an ultra-dry air atmosphere (inwhich the content of water is lower than or equal to 20 ppm, preferablylower than or equal to 1 ppm, more preferably lower than or equal to 10ppb) at a temperature higher than or equal to 450° C. and lower than orequal to 850° C., preferably higher than or equal to 550° C. and lowerthan or equal to 750° C. so that water or hydrogen that serves as ann-type impurity is removed from the oxide semiconductor. By highlypurifying the oxide semiconductor by removal of an impurity such aswater or hydrogen, the carrier density can be lower than 1×10¹² cm⁻³,preferably lower than 1.45×10¹⁰ cm⁻³, i.e., lower than the lower limitof measurement.

Further, the heat treatment is performed at a temperature higher than orequal to 450° C. and lower than or equal to 850° C., preferably higherthan or equal to 600° C. and lower than or equal to 700° C., in whichcase the oxide semiconductor can be highly purified and crystallized soas to be an oxide semiconductor having a c-axis aligned polycrystallineregion in which crystals grow from a surface of the oxide semiconductorto the inside portion thereof. The c-axis aligned polycrystalline regionis a region which has a plurality of single crystal regions in each ofwhich the c-axis is aligned to the perpendicular direction to thesurface.

In an embodiment of the present invention, the second oxidesemiconductor film is provided using the oxide semiconductor film havingthe c-axis aligned polycrystalline region as a seed crystal, and heattreatment is performed at a temperature higher than or equal to 450° C.and lower than or equal to 850° C., preferably higher than or equal to550° C. and lower than or equal to 750° C., whereby the second oxidesemiconductor film can have a polycrystalline region which is c-axisaligned like the seed crystal. That is, ideal axial growth or epitaxialgrowth can be caused in which the direction of the c-axis of the seedcrystal and that of the c-axis of the second oxide semiconductor filmare identical.

The second oxide semiconductor film the direction of c-axis of which isidentical to that of the c-axis of the seed crystal can be obtained notonly by solid-phase growth caused by the heat treatment after depositionbut also by being deposited, typically sputtered, while heating isperformed at a temperature higher than or equal to 200° C. and lowerthan or equal to 600° C., preferably higher than or equal to 200° C. andlower than or equal to 550° C., where crystal growth can be caused inthe second oxide semiconductor film while the second oxide semiconductorfilm is deposited.

Furthermore, in a transistor, an oxide semiconductor functions as a pathin which carriers flow by reducing or preferably eliminating carriers ofthe oxide semiconductor. As a result, the oxide semiconductor is ani-type (intrinsic) semiconductor which is highly purified and includesextremely small number of or no carriers, and off-state current can beextremely small in the state where the transistor is in an off-state,which is the technical idea of this embodiment.

In addition, when the oxide semiconductor functions as a path, and theoxide semiconductor itself is an i-type (intrinsic) semiconductor whichis highly purified so as to include extremely small number of or nocarriers, carriers are supplied from source and drain electrodes. Byappropriately selecting the electron affinity (χ) of the oxidesemiconductor, the Fermi level which may ideally correspond to theintrinsic Fermi level, and the work function of a material of the sourceand drain electrodes, carriers can be injected from the source and drainelectrodes so that an n-channel transistor and a p-channel transistorcan be manufactured as appropriate.

On the other hand, a horizontal transistor in which a channel is formedsubstantially in parallel with a substrate needs a source and a drain aswell as the channel, so that an area occupied by the transistor in thesubstrate is increased, which hinders miniaturization. However, asource, a channel, and a drain are stacked in a vertical transistor,whereby an occupation area in a substrate surface can be reduced. As aresult of this, it is possible to miniaturize the transistor.

As described above, the oxide semiconductor film is highly purified sothat an impurity that is not a main component of the oxide semiconductorfilm, typically hydrogen, water, hydroxy group, or hydride, may becontained as little as possible, and the oxide semiconductor film ismade to have the polycrystalline region, whereby good operation of thetransistor can be obtained. In particular, withstand voltage can behigher, a short channel effect can be reduced, and a high on-off ratiocan be obtained. In addition, the amount of shift in a threshold voltageof the transistor between before and after a BT test can be suppressed,whereby high reliability can be obtained. Temperature dependence ofelectrical characteristics can also be suppressed. Further, an oxidesemiconductor film having a polycrystalline region can be formed thickerat a relatively low temperature by the above-described method in which aflat-plate-shaped polycrystalline region is formed in an oxidesemiconductor film and then crystal growth is caused with the use of thepolycrystalline region as a seed crystal, in spite of the fact that anymetal oxide which has been already reported is in an amorphous state ora polycrystalline state or can be in a single crystal state only by atreatment at a temperature as high as around 1400° C. Therefore, a widerindustrial application can be realized.

Embodiment 2

In this embodiment, a transistor having a structure different from thatof Embodiment 1 will be described with reference to FIGS. 7A and 7B.

FIG. 7A is a top view of a transistor 147, and FIG. 7B corresponds to across-sectional view taken along dashed line A-B in FIG. 7A.

As illustrated in FIG. 7B, the first electrode 105, the oxidesemiconductor film 107, and the second electrode 109 are stacked overthe insulating film 103 formed over the substrate 101. Note that thisstructure is different from that in Embodiment 1 in that the peripheryof the second electrode 109 is inside the periphery of the oxidesemiconductor film 107. The gate insulating film 111 is provided so asto cover the first electrode 105, the oxide semiconductor film 107, andthe second electrode 109. Over the gate insulating film 111, the thirdelectrode 113 is provided so as to face at least side surfaces of theoxide semiconductor film and the second electrode. The insulating film117 functioning as an interlayer insulating film is provided over thegate insulating film 111 and the third electrode 113. Openings areformed in the insulating film 117, and the wiring 131 (see FIG. 7A)connected through the opening to the first electrode 105, the wiring 129connected through the opening to the second electrode 109, and thewiring 125 connected through the opening to the third electrode 113 areformed.

In this embodiment, as in Embodiment 1, the oxide semiconductor film 107has crystallinity and the polycrystalline region of the oxidesemiconductor film 107 is c-axis aligned in a direction perpendicular tothe surface. That is, the c-axis direction of the oxide semiconductorfilm 107 corresponds to the direction perpendicular to the surface. Notethat elements which are adjacent to each other in the a-b plane are ofthe same kind.

The oxide semiconductor film 107 is highly purified and the hydrogenconcentration therein is lower than or equal to 1×10¹⁸ cm⁻³, preferablylower than or equal to 1×10¹⁶ cm⁻³, more preferably substantially 0. Thecarrier density of the oxide semiconductor film 107 is lower than 1×10¹²cm⁻³, preferably lower than 1.45×10¹⁰ cm⁻³, which is lower than thelower limit of measurement. That is, the carrier density of the oxidesemiconductor film is as close to zero as possible. A band gap isgreater than or equal to 2 eV, preferably greater than or equal to 2.5eV, more preferably greater than or equal to 3 eV.

In the transistor of this embodiment, the channel length L means adistance between the first electrode 105 and the second electrode 109 ina region of the oxide semiconductor which is in contact with the gateinsulating film in a cross-sectional structure. Further, the channelwidth W means the length between the end portions of the oxidesemiconductor film that are in contact with the first electrode or thesecond electrode. Note that, here, the length of the end portion of theoxide semiconductor film that is in contact with one of the firstelectrode and the second electrode which has a larger area than theother is W. In this embodiment, since the shape of the top surface ofthe oxide semiconductor film of the transistor is rectangular, thechannel width W is the sum of 2W₁ and 2W₂. Note that in the case wherethe shape of the top surface of the oxide semiconductor film of thetransistor is circular, the channel width W is 2πr where r is a radiusof the circle.

In this embodiment, when compared with Embodiment 1, the channel lengthL is larger. In addition, there is influence of voltage applied to thethird electrode 113 functioning as a gate electrode not only on the sidesurface of the oxide semiconductor film 107 but also on the top surfaceof the oxide semiconductor film 107. Thus, a channel can be more easilycontrolled than in Embodiment 1.

As described above, the oxide semiconductor film is highly purified sothat an impurity that is not a main component of the oxide semiconductorfilm, typically hydrogen, water, hydroxy group, or hydride, may becontained as little as possible, and is made to have the polycrystallineregion, whereby good operation of the transistor can be obtained. Inparticular, withstand voltage can be higher, a short channel effect canbe reduced, and a high on-off ratio can be realized. In addition, theamount of shift in a threshold voltage of the transistor between beforeand after the BT test can be suppressed, whereby high reliability can beobtained. Temperature dependence of electrical characteristics can alsobe suppressed.

Embodiment 3

In this embodiment, a structure of a three-terminal type diode which ismanufactured using the transistor described in Embodiment 1 or 2 will bedescribed with reference to FIGS. 8A and 8B.

FIGS. 8A and 8B are each a cross-sectional view of a three-terminal typediode.

In a three-terminal type diode 149 a illustrated in FIG. 8A, the firstelectrode 105, the oxide semiconductor film 107, and the secondelectrode 109 are stacked over the insulating film 103 formed over thesubstrate 101. The gate insulating film 111 is provided so as to coverthe first electrode 105, the oxide semiconductor film 107, and thesecond electrode 109. The third electrode 113 is provided over the gateinsulating film 111. Further, an opening is formed in the gateinsulating film 111 and the second electrode 109 and the third electrode113 are connected to each other through the opening.

In a three-terminal type diode 149 b illustrated in FIG. 8B, the firstelectrode 105, the oxide semiconductor film 107, and the secondelectrode 109 are stacked over the insulating film 103 formed over thesubstrate 101. The gate insulating film 111 is provided so as to coverthe first electrode 105, the oxide semiconductor film 107, and thesecond electrode 109. The third electrode 113 is provided over the gateinsulating film 111. Further, an opening is formed in the gateinsulating film 111 and the first electrode 105 and the third electrode113 are connected to each other through the opening.

In each of the three-terminal type diodes described in this embodiment,the third electrode functioning as a gate electrode and one of a sourceelectrode and a drain electrode are electrically connected. For example,in the case where the first electrode functioning as the drain electrodeand the third electrode functioning as the gate electrode areelectrically connected to each other, when voltage (a positive voltage)that is higher than that of the source electrode is applied to the drainelectrode, a positive voltage is also applied to the gate electrode;thus, the transistor is turned on and forward current flows more easily.On the other hand, when voltage (a negative voltage) that is lower thanthat of the source electrode is applied to the drain electrode, thetransistor is turned off and reverse current flows with more difficulty.Accordingly, a rectification property of the diode can be enhanced.

Note that although three-terminal type diodes are described in thisembodiment, two-terminal type diodes which do not have the thirdelectrodes can also be manufactured.

Embodiment 4

In this embodiment, transistors with high heat resistance will bedescribed with reference to FIGS. 1A and 1B.

By using a substrate having a high heat-dissipation property as thesubstrate 101 illustrated in FIGS. 1A and 1B, a transistor with highheat resistance can be manufactured. Examples of the substrate having ahigh heat-dissipation property include a semiconductor substrate, ametal substrate, a plastic substrate, and the like. As typical examplesof the semiconductor substrate, a single crystal semiconductor substratesuch as a silicon substrate or a silicon carbide substrate, apolycrystalline semiconductor substrate, a compound semiconductorsubstrate such as a silicon germanium substrate, and the like can begiven. As typical examples of the metal substrate, an aluminumsubstrate, a copper substrate, a stainless steel substrate, and the likecan be given. As a typical example of the plastic substrate, a plasticsubstrate containing a carbon fiber, a metal fiber, a metal piece, orthe like can be given. Note that the semiconductor substrate, the metalsubstrate, and the plastic substrate are not limited to the abovesubstrates, and any substrate can be used as appropriate as long as ithas a high heat-dissipation property.

By using an insulating film having high thermal conductivity as theinsulating film 103 illustrated in FIGS. 1A and 1B, a transistor withhigh heat resistance can be manufactured. Examples of the insulatingfilm having high thermal conductivity include an aluminum nitride film,an aluminum nitride oxide film, a silicon nitride film, and the like.

A semiconductor film may be formed between the first electrode 105 andthe insulating film 103 illustrated in FIGS. 1A and 1B. As typicalexamples of the semiconductor film, a silicon film, a germanium film, asilicon carbide film, a diamond like carbon (DLC) film, and the like canbe given.

Note that by using one or more of the above components, a transistorwith high heat resistance can be manufactured.

Embodiment 5

In this embodiment, a transistor including the first electrode 105 andthe second electrode 109 that are formed using materials havingdifferent work functions will be described.

In this embodiment, one of the first electrode 105 and the secondelectrode 109 is formed using a conductive material having a workfunction that is lower than or equal to the electron affinity of anoxide semiconductor, and the other of the first electrode 105 and thesecond electrode 109 is formed using a conductive material having a workfunction that is higher than the electron affinity of the oxidesemiconductor.

For example, in the case where the electron affinity (χ) of the oxidesemiconductor is 4.3 eV, as the conductive material having a workfunction that is higher than the electron affinity of the oxidesemiconductor, tungsten (W), molybdenum (Mo), chromium (Cr), iron (Fe),indium tin oxide (ITO), or the like can be used. As the conductivematerial having a work function that is lower than or equal to theelectron affinity of the oxide semiconductor, titanium (Ti), yttrium(Y), aluminum (Al), magnesium (Mg), silver (Ag), zirconium (Zr), or thelike can be used.

First, described is a case where an electrode functioning as a drain isformed using a conductive material having a work function that is higherthan the electron affinity of the oxide semiconductor, and an electrodefunctioning as a source is formed using a conductive material having awork function that is lower than or equal to the electron affinity ofthe oxide semiconductor.

The relation among the work function of the conductive material forforming the electrode functioning as a drain φ_(md), the work functionof the conductive material for forming the electrode functioning as asource φ_(ms), and the electron affinity χ is set so as to be expressedas Formula 6.

φms≦χ≦φmd   [Formula 6

As can be seen, the work function of the conductive material of theelectrode functioning as a source is lower than or equal to the electronaffinity of the oxide semiconductor; therefore, a barrier in an on-stateof the transistor (for example, h in FIG. 3B) can be reduced, thetransistor can be turned on at a low gate voltage, and a large amount ofcurrent can flow.

In another case, the relation between the work function φ_(md), theelectron affinity χ, and the work function φ_(ms) is set so as to beexpressed as Formula 7

.

φmd≦χ≦φms   [Formula 7

As can be seen, since the work function of the conductive material ofthe electrode functioning as a source is higher than the electronaffinity of the oxide semiconductor, the barrier of the transistorbecomes high. Accordingly, the amount of current in an off-state can bereduced.

Note that the electrode functioning as a source can be one of the firstelectrode 105 and the second electrode 109, and the electrodefunctioning as a drain can be the other of the first electrode 105 andthe second electrode 109.

From the above, by forming one of the first electrode 105 and the secondelectrode 109 using a conductive material having a work function that islower than or equal to the electron affinity of the oxide semiconductorand by forming the other of the first electrode 105 and the secondelectrode 109 using a conductive material having a work function that ishigher than the electron affinity of the oxide semiconductor, on-statecharacteristics or off-state characteristics of a transistor can beimproved.

In addition, the rectification property of the diode described inEmbodiment 3 can also be enhanced by satisfying Formula 6 or 7.

Embodiment 6

In this embodiment, a manufacturing process of the transistorillustrated in FIGS. 1A and 1B or FIGS. 7A and 7B will be described withreference to FIGS. 9A to 9E, FIGS. 10A to 10C, FIGS. 11A and 11B, andFIGS. 12A to 12C.

As illustrated in FIG. 9A, the insulating film 103 is formed over thesubstrate 101, and the first electrode 105 is formed over the insulatingfilm 103. The first electrode 105 functions as one of the sourceelectrode and the drain electrode of the transistor.

The insulating film 103 can be formed by a sputtering method, a CVDmethod, a coating method, or the like.

Note that when the insulating film 103 is formed by a sputtering method,the insulating film 103 is preferably formed while hydrogen, water,hydroxy group, hydride, or the like remaining in a treatment chamber isremoved. This is for preventing hydrogen, water, hydroxy group, hydride,or the like from being contained in the insulating film 103. Anentrapment vacuum pump is preferably used for removing hydrogen, water,hydroxy group, hydride, or the like remaining in the treatment chamber.For example, a cryopump, an ion pump, or a titanium sublimation pump ispreferably used as the entrapment vacuum pump. The evacuation unit canbe a turbo pump provided with a cold trap. Since hydrogen, water,hydroxy group, hydride, or the like are evacuated in the treatmentchamber which is evacuated using a cryopump, in the insulating film 103formed in the treatment chamber, the concentration of an impuritycontained in the insulating film 103 can be reduced.

As a sputtering gas used for formation of the insulating film 103, ahigh-purity gas from which an impurity such as hydrogen, water, hydroxygroup, or hydride is removed to such a level that the impurityconcentration is represented by the unit “ppm” or “ppb”.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used for a sputtering power supply, aDC sputtering method, and a pulsed DC sputtering method in which a biasis applied in a pulsed manner. The RF sputtering method is mainly usedin the case where an insulating film is formed, and the DC sputteringmethod is mainly used in the case where a metal film is formed.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can beformed to be stacked in the same chamber, or films of plural kinds ofmaterials can be formed by electric discharge at the same time in thesame chamber.

Alternatively, a sputtering apparatus provided with a magnet systeminside the chamber and used for a magnetron sputtering method, or asputtering apparatus used for an ECR sputtering method in which plasmagenerated with the use of microwaves is used without using glowdischarge can be used.

Further, as a sputtering method, a reactive sputtering method in which atarget substance and a sputtering gas component are chemically reactedwith each other during deposition to form a thin compound film thereof,or a bias sputtering method in which voltage is also applied to asubstrate during deposition can be used.

As the sputtering in this specification, the above-described sputteringapparatus and the sputtering method can be employed as appropriate.

In this embodiment, the substrate 101 is carried into the treatmentchamber. A sputtering gas containing high purity oxygen, from whichhydrogen, water, hydroxy group, hydride, or the like is removed, isintroduced into the treatment chamber, and a silicon oxide film isformed as the insulating film 103 over the substrate 101 using a silicontarget. Note that when the insulating film 103 is formed, the substrate101 may be heated.

For example, a silicon oxide film is formed with an RF sputtering methodin the following conditions: a quartz (preferably, synthetic quartz)target is used; the substrate temperature is 108° C.; the distancebetween the substrate and the target (the T—S distance) is 60 mm; thepressure is 0.4 Pa; the high frequency power is 1.5 kW; and theatmosphere is an atmosphere containing oxygen and argon (the flow ratioof oxygen to argon is 1:1 (each flow rate is 25 sccm)). The filmthickness may be 100 nm. Note that instead of a quartz (preferably,synthesized quartz) target, a silicon target can be used. Note thatoxygen or a mixed gas of oxygen and argon is used as a sputtering gas.

For example, when the insulating film 103 is formed to have astacked-layer structure, a silicon nitride film is formed using asilicon target and a sputtering gas containing high purity nitrogen fromwhich hydrogen, water, hydroxy group, hydride, or the like is removed,between the silicon oxide film and the substrate. Also in this case, asin the case of the silicon oxide film, it is preferable to deposit asilicon nitride film while hydrogen, water, hydroxy group, hydride, orthe like remaining in the treatment chamber is removed. Note that inthis step, the substrate 101 may be heated.

In the case where a silicon nitride film and a silicon oxide film arestacked as the insulating film 103, the silicon nitride film and thesilicon oxide film can be deposited in the same treatment chamber withthe use of a common silicon target. First, the silicon nitride film isformed in such a manner that an etching gas containing nitrogen isintroduced and a silicon target mounted on the treatment chamber isused. Then, the silicon oxide film is formed in such a manner that thegas is switched to an etching gas containing oxygen and the same silicontarget is used. The silicon nitride film and the silicon oxide film canbe formed successively without exposure to the air; thus, adsorption ofan impurity such as hydrogen, water, hydroxy group, or hydride on asurface of the silicon nitride film can be prevented.

The first electrode 105 can be formed in such a manner that a conductivefilm is formed over the substrate 101 by a sputtering method, a CVDmethod, or a vacuum evaporation method, a resist mask is formed over theconductive film in a photolithography step, and the conductive film isetched using the resist mask. Alternatively, the first electrode 105 isformed by a printing method or an ink-jet method without using aphotolithography step, so that the number of steps can be reduced. Notethat end portions of the first electrode 105 preferably have a taperedshape, so that the coverage with a gate insulating film to be formedlater is improved. When the angle formed between the end portion of thefirst electrode 105 and the insulating film 103 is greater than or equalto 30° and less than or equal to 60° (preferably, greater than or equalto 40° and less than or equal to 50°), the coverage with the gateinsulating film to be formed later can be improved.

In this embodiment, as the conductive film to serve as the firstelectrode 105, a titanium film is formed to a thickness of 50 nm by asputtering method, an aluminum film is formed to a thickness of 100 nm,and a titanium film is formed to a thickness of 50 nm. Next, etching isperformed using the resist mask formed in the photolithography step,whereby the first electrode 105 is formed.

Next, as illustrated in FIG. 9B, a first oxide semiconductor film 102 awith a thickness greater than or equal to 2 nm and less than or equal to15 nm is formed over the insulating film 103 and the first electrode105.

Here, a method for manufacturing the first oxide semiconductor film 102a will be described.

Over the insulating film 103 and the first electrode 105, the firstoxide semiconductor film 102 a is formed to a thickness greater than orequal to 2 nm and less than or equal to 15 nm by a sputtering method, acoating method, a printing method, or the like.

The first oxide semiconductor film 102 a can be formed by a sputteringmethod in a rare gas (typically, argon) atmosphere, an oxygenatmosphere, or an atmosphere including a rare gas (typically, argon) andoxygen.

In addition, it is preferable that moisture or the like which remains inthe sputtering apparatus is removed before, during, or after depositionof the first oxide semiconductor film 102 a. In order to remove theremaining moisture in the sputtering apparatus, an entrapment vacuumpump is preferably used. For example, a cryopump, an ion pump, or atitanium sublimation pump is preferably used as the entrapment vacuumpump. The evacuation unit can be a turbo pump provided with a cold trap.From a film formation chamber of the sputtering apparatus in whichexhaustion is performed with the use of a cryopump, a hydrogen atom, acompound including a hydrogen atom such as water (H₂O), or the like, forexample, is exhausted. Accordingly, the concentration of an impuritycontained in the oxide semiconductor film formed in the film formationchamber can be reduced.

As the first oxide semiconductor film 102 a, any one of the followingmetal oxide films can be used: a four-component metal oxide film such asan In—Sn—Ga—Zn—O film; a three-component metal oxide film such as anIn—Ga—Zn—O film, an In—Sn—Zn—O film, an In—Al—Zn—O film, a Sn—Ga—Zn—Ofilm, an Al—Ga—Zn—O film, and a Sn—Al—Zn—O film; a two-component metaloxide film such as an In—Zn—O film, a Sn—Zn—O film, an Al—Zn—O film, aZn—Mg—O film, a Sn—Mg—O film, and an In—Mg—O film; an In—O film; a Sn—Ofilm; and a Zn—O film.

For the first oxide semiconductor film 102 a, a material expressed byInMO₃(ZnO)_(m) (m>0) can be used. Here, M represents one or more metalelements selected from Ga, Al, Mn, and Co. For example, M can be Ga, Gaand Al, Ga and Mn, Ga and Co, or the like.

Note also that the first oxide semiconductor film 102 a may be formedusing an oxide semiconductor material represented by In-A-B—O. Here, Arepresents one or more kinds of elements selected from Group 13 elementssuch as gallium (Ga) and aluminum (Al), Group 14 elements such assilicon (Si) and germanium (Ge), and the like. B represents one or morekinds of elements selected from Group 12 elements such as zinc (Zn). Itis to be noted that values of an In content, an A content, and a Bcontent are arbitrary. The value of the A content may be 0. On the otherhand, the value of the In content and that of the B content are not 0.In other words, the above expression may represent In—Ga—Zn—O, In—Zn—O,and the like.

When the first oxide semiconductor film is formed, a metal oxide targethaving a composition ratio of In:Ga:Zn=1: greater than or equal to 0 andless than or equal to 2: greater than or equal to 1 and less than orequal to 5 is used. In this embodiment, the first oxide semiconductorfilm is formed to a thickness of 5 nm in an oxygen atmosphere, an argonatmosphere, or a mixed atmosphere of argon and oxygen in the followingconditions: an oxide semiconductor target (an In—Ga—Zn—O-based oxidesemiconductor target (In₂O₃:Ga₂O₃:ZnO =1:1:2 [molar ratio] andIn:Ga:Zn=1:1:1 [atom ratio])) is used; the distance between thesubstrate and the target is 170 mm, pressure is 0.4 Pa; and a directcurrent (DC) power source is 0.5 kW. Alternatively, as an oxidesemiconductor target, a target having a composition ratio ofIn:Ga:Zn=1:1:0.5 [atom ratio], a target having a composition ratio ofIn:Ga:Zn=1:1:1 [atom ratio], a target having a composition ratio ofIn:Ga:Zn=1:1:2 [atom ratio], or a target having a composition ratio ofIn:Ga:Zn=1:0.5:2 [atom ratio] can be used. Further alternatively, atarget having a composition ratio of In:Zn=1:1 [atom ratio], which doesnot contain Ga may be used. Field-effect mobility can be higher in thecase where an In—Zn—O film is used than in the case where an In—Ga—Zn—Ofilm is used. In this embodiment, since crystallization is intentionallycaused by performing heat treatment in a later step, it is preferable touse an oxide semiconductor target in which crystallization is easilycaused.

It is preferable that the relative density of the oxide semiconductor inthe oxide semiconductor target is greater than or equal to 80%, morepreferably greater than or equal to 95%, further preferably greater thanor equal to 99.9%. When a target having a high relative density is used,the impurity concentration in an oxide semiconductor film to be formedcan be reduced, so that a transistor with excellent electricalcharacteristics or high reliability can be obtained.

Further, preheat treatment is preferably performed before the firstoxide semiconductor film 102 a is formed, in order to remove moisture orhydrogen which remains on an inner wall of a sputtering apparatus, on asurface of the target, or inside a target material. As the preheattreatment, a method in which the inside of the film formation chamber isheated to 200° C. to 600° C. under reduced pressure, a method in whichintroduction and exhaust of nitrogen or an inert gas are repeated whilethe inside of the film formation chamber is heated, and the like can begiven. After the preheat treatment, the substrate or the sputteringapparatus is cooled. Then, an oxide semiconductor film is formed withoutexposure to the air. Although a certain level of effect can be obtainedwhen introduction and exhaust of nitrogen are repeated without heating,it is more preferable to perform the treatment with the inside of thefilm formation chamber heated.

Then, the first oxide semiconductor film 102 a is subjected to the firstheat treatment and at least part of the oxide semiconductor film iscrystallized. The first heat treatment is performed at a temperaturehigher than or equal to 450° C. and lower than or equal to 850° C.,preferably higher than or equal to 550° C. and lower than or equal to750° C. Heating time is greater than or equal to 1 minute and less thanor equal to 24 hours. By the first heat treatment, a first oxidesemiconductor film 102 b (also referred to as a first oxidesemiconductor film having crystallinity) which has a polycrystallineregion growing from the surface is formed (see FIG. 9C). Thepolycrystalline region grows from the surface to the inside portion andcontains plate-like crystals whose average thickness is greater than orequal to 2 nm and less than or equal to 15 nm The polycrystalline regionformed at the surface is c-axis aligned in a direction perpendicular tothe surface. In this embodiment, an example is described in which mostpart of the first oxide semiconductor film is made to contain apolycrystal by the first heat treatment. The polycrystalline regionhaving relatively uniform crystal alignment which is formed at thesurface of the first oxide semiconductor film grows from the surface inthe perpendicular direction; thus, the polycrystalline region can beformed without being affected by a base member.

Note that a crystal grain boundary exists in a region of the first oxidesemiconductor film 102 b which overlaps with a projected portion and arecessed portion made by the first electrode 105 and a polycrystal iscontained in the region. Although the a-b plane, a-axis, and b-axis ofeach crystal in the polycrystal may not be identical in the first oxidesemiconductor film 102 b.

An example of a mechanism of formation of a crystal region havinguniform crystal alignment at the surface of the first oxidesemiconductor film in the case where the first oxide semiconductor filmis, for example, an In—Ga—Zn—O film is described. By heat treatment,zinc contained in the In—Ga—Zn—O film diffuses and gathers in thevicinity of the surface so as to be a seed crystal. In the crystalgrowth at this time, crystals grow more in a direction parallel to thesurface than in a direction perpendicular to the surface so that aflat-plate-shaped polycrystalline region is formed. In other words,crystallization is caused more easily in the a-b plane direction than inthe c-axis direction. Further, in the flat-plate-shaped polycrystallineregion, the a-b planes of the single crystal regions become parallel tothe surface. In addition, a free space is located above the surface ofthe In—Ga—Zn—O film and crystals do not grow upward in the free space.As for these facts, the fact that when the In—Ga—Zn—O film was measuredwith thermal desorption spectroscopy (TDS) while the temperature wasincreased to 450° C., peaks of In and Ga were not detected but a peak ofzinc was detected in a vacuum heating condition, particularly at around300° C. is observed. Note that the TDS measurement was performed in avacuum and it was observed that zinc was detected from around 200° C.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in nitrogen, oxygen or a raregas such as helium, neon, or argon. In addition, nitrogen, oxygen, or arare gas such as helium, neon, or argon which is introduced into a heattreatment apparatus preferably has a purity of 6 N (99.9999%) or higher,more preferably 7 N (99.99999%) or higher (that is, the concentration ofan impurity is lower than or equal to 1 ppm, preferably lower than orequal to 0.1 ppm). Further, the first heat treatment may be performed inan ultra-dry air atmosphere in which the content of water is lower thanor equal to 20 ppm, preferably lower than or equal to 1 ppm, morepreferably lower than or equal to 10 ppb.

In this embodiment, heat treatment in a dry air atmosphere at 700° C.for 1 hour is performed as the first heat treatment.

In addition, at the time of increasing the temperature in the first heattreatment, an atmosphere of a furnace may be a nitrogen atmosphere andthe atmosphere may be switched to an oxygen atmosphere at the time ofperforming cooling. The inside portion of the first oxide semiconductorfilm can be supplied with oxygen so as to be an i-type oxidesemiconductor film by switching the atmosphere to the oxygen atmosphereafter the dehydration or dehydrogenation is performed in the nitrogenatmosphere.

Note that the heat treatment apparatus for the first heat treatment isnot limited to a particular apparatus, and the apparatus may be providedwith a device for heating an object to be processed by heat radiation orheat conduction from a heating element such as a resistance heatingelement. For example, an electric furnace, or a rapid thermal annealing(RTA) apparatus such as a gas rapid thermal annealing (GRTA) apparatusor a lamp rapid thermal annealing (LRTA) apparatus can be used. An LRTAapparatus is an apparatus for heating an object to be processed byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas.

Next, over the first oxide semiconductor film 102 b which has theflat-plate-shaped polycrystalline region at least at the surfacethereof, a second oxide semiconductor film 104 a whose thickness islarger than that of the first oxide semiconductor film 102 b is formed(see FIG. 9D). The thickness of the second oxide semiconductor film 104a is greater than or equal to 1 μm, preferably greater than or equal to3 μm, more preferably greater than or equal to 10 μm. The second oxidesemiconductor film 104 a can be formed by a sputtering method in a raregas (typically, argon) atmosphere, an oxygen atmosphere, or anatmosphere including a rare gas (typically, argon) and oxygen.

As the second oxide semiconductor film 104 a, any one of the followingmetal oxide films can be used: a four-component metal oxide film such asan In—Sn—Ga—Zn—O film; a three-component metal oxide film such as anIn—Ga—Zn—O film, an In—Sn—Zn—O film, an In—Al—Zn—O film, a Sn—Ga—Zn—Ofilm, an Al—Ga—Zn—O film, and a Sn—Al—Zn—O film; a two-component metaloxide film such as an In—Zn—O film, a Sn—Zn—O film, an Al—Zn—O film, aZn—Mg—O film, a Sn—Mg—O film, and an In—Mg—O film; an In—O film; a Sn—Ofilm; and a Zn—O film.

Further, it is preferable that a material of the first oxidesemiconductor film and that of the second oxide semiconductor film 104 acontain the same components. In the case of using the materialscontaining the same components, crystal growth is easily caused with theuse of the polycrystalline region of the first oxide semiconductor filmas a seed crystal in crystallization which is performed later. Inaddition, when the materials contain the same components, physicalproperties of an interface such as adhesion or electricalcharacteristics are favorable.

After that, by performing second heat treatment, crystal growth iscaused with the use of the single crystal region of the first oxidesemiconductor film 102 b as a seed crystal. The second heat treatment isperformed at a temperature higher than or equal to 450° C. and lowerthan or equal to 850° C., preferably higher than or equal to 600° C. andlower than or equal to 700° C. Heating time is greater than or equal to1 minute and less than or equal to 24 hours. By the second heattreatment, the second oxide semiconductor film 104 a is crystallized.Thus, an oxide semiconductor film 108 (also referred to as a secondoxide semiconductor film having crystallinity) which has apolycrystalline region can be obtained (see FIG. 9E). At this time, itis preferable that crystals in the oxide semiconductor film 108 have thesame structure and close lattice constants (lattice mismatch is lessthan or equal to 1%). Note that the oxide semiconductor film 108includes the first oxide semiconductor film and the second oxidesemiconductor film. When the second heat treatment is performed at atemperature higher than or equal to 450° C. and lower than or equal to850° C., preferably higher than or equal to 600° C. and lower than orequal to 700° C., crystal growth can be caused in the second oxidesemiconductor film 104 a (also referred to as epitaxial growth or axialgrowth) so that a crystal axis in the crystal growth in the second oxidesemiconductor film 104 a and a crystal axis of the first oxidesemiconductor film 102 b are substantially identical. Further, epitaxialor axial growth can be caused in the second oxide semiconductor film 104a by solid phase growth.

The steps illustrated in FIGS. 9C to 9E are specifically described withreference to FIG. 12A to 12C.

In FIG. 12A, the first oxide semiconductor film 102 b after beingsubjected to the first heat treatment for crystallization isillustrated. FIG. 12A corresponds to FIG. 9C. FIG. 12B, whichcorresponds to FIG. 9D, is a cross-sectional view of the second oxidesemiconductor film 104 a just after being deposited. FIG. 12C, whichcorresponds to FIG. 9E, is a cross-sectional view after the second heattreatment. By the second heat treatment, the oxide semiconductor film108 which has a polycrystalline region having higher alignment can beobtained. In the case where an oxide semiconductor material of the firstoxide semiconductor film and that of the second oxide semiconductor filmcontain the same main components, as illustrated in FIG. 12C, upwardcrystal growth proceeds to the surface of the second oxide semiconductorfilm 104 b with the use of the single crystal region of the first oxidesemiconductor film 102 b as a seed crystal, so that the second oxidesemiconductor film 104 b is formed, and the oxide semiconductor filmshave the same crystal structure. Therefore, although indicated by adotted line in FIG. 12C, a boundary between the first oxidesemiconductor film and the second oxide semiconductor film may becomeunclear. Furthermore, by the second heat treatment, the inside portionof the second oxide semiconductor film 104 b just after deposition ishighly purified.

Note that the second heat treatment is performed in a nitrogenatmosphere, an oxygen atmosphere, or a rare gas atmosphere such ashelium, neon, or argon. At this time, it is preferable that water,hydrogen, and the like be not contained in nitrogen, oxygen or a raregas such as helium, neon, or argon. In addition, nitrogen, oxygen, or arare gas such as helium, neon, or argon which is introduced into a heattreatment apparatus preferably has a purity of 6 N or higher, morepreferably 7 N or higher (that is, the concentration of an impurity islower than or equal to 1 ppm, preferably lower than or equal to 0.1ppm). Further, the second heat treatment may be performed in anultra-dry air atmosphere in which the content of water is lower than orequal to 20 ppm, preferably lower than or equal to 1 ppm. In addition,at the time of increasing the temperature in the second heat treatment,an atmosphere of a furnace may be a nitrogen atmosphere and theatmosphere may be switched to an oxygen atmosphere at the time ofperforming cooling.

Note that the heat treatment apparatus for the second heat treatment isnot limited to a particular apparatus, and the apparatus may be providedwith a device for heating an object to be processed by heat radiation orheat conduction from a heating element such as a resistance heatingelement. For example, an electric furnace, or an RTA apparatus such as aGRTA apparatus or an LRTA apparatus can be used.

Next, after a resist mask is formed in a photolithography step over theoxide semiconductor film 108 including the first and second oxidesemiconductor films, the oxide semiconductor film 108 is etched usingthe resist mask, whereby the island-shaped oxide semiconductor film 107is formed. The resist mask for forming the island-shaped oxidesemiconductor film 107 may be formed by an ink-jet method. No photomaskis used when a resist mask is formed by an ink-jet method; therefore,production cost can be reduced. The angle formed between the firstelectrode 105 and the end portions of the second electrode 109 and theoxide semiconductor film 107 is set to greater than or equal to 30° andless than or equal to 60°, preferably greater than or equal to 40° andless than or equal to 50° by this etching, whereby the coverage with thegate insulating film that is formed later can be improved.

Note that the etching of the oxide semiconductor film here may beperformed by dry etching, wet etching, or both wet etching and dryetching. In order to form the oxide semiconductor film 107 with adesired shape, the etching conditions (an etchant, etching time,temperature, and the like) are adjusted as appropriate depending on thematerial.

When the etching rate of the oxide semiconductor film is different fromthat of the first electrode 105, a condition such that the etching rateof the first electrode 105 is low and the etching rate of the oxidesemiconductor film is high is selected.

As an etchant used for wet etching of the oxide semiconductor film, amixed solution of phosphoric acid, acetic acid, and nitric acid, anammonia hydrogen peroxide mixture (hydrogen peroxide water at 31 wt %:ammonia water at 28 wt %: water=5:2:2), or the like can be used. Inaddition, ITO07N (produced by KANTO CHEMICAL CO., INC.) may also beused.

The etchant after the wet etching is removed together with the etchedmaterials by cleaning. The waste liquid including the etchant and thematerial etched off may be purified and the material may be reused.Materials such as indium contained in the oxide semiconductor film arecollected from the waste liquid after the etching and recycled, so thatresources can be effectively used and cost can be reduced.

As an etching gas used for dry etching of the oxide semiconductor film,a gas containing chlorine (a chlorine-based gas such as chlorine (Cl₂),boron trichloride (BCl₃), silicon tetrachloride (SiCl₄), or carbontetrachloride (CCl₄)) is preferably used.

Alternatively, a gas containing fluorine (a fluorine-based gas such ascarbon tetrafluoride (CF₄), sulfur hexafluoride (SF₆), nitrogentrifluoride (NF₃), or trifluoromethane (CHF₃)); hydrogen bromide (HBr);oxygen (O₂); any of these gases to which a rare gas such as helium (He)or argon (Ar) is added; or the like can be used.

As the dry etching method, a parallel plate RIE (reactive ion etching)method or an ICP (inductively coupled plasma) etching method can beused. In order to etch the films into desired shapes, the etchingconditions (the amount of electric power applied to a coil-shapedelectrode, the amount of electric power applied to an electrode on asubstrate side, the temperature of the electrode on the substrate side,or the like) are adjusted as appropriate.

Then, over the insulating film, the first electrode 105, and theisland-shaped oxide semiconductor film 107, a conductive film 110 to bethe second electrode 109 is formed (see FIG. 10A). The conductive film110 serves as the second electrode 109 later. The conductive film 110can be formed by appropriately using the material and method for thefirst electrode 105; when a material having an etching rate which ishigher than that of the first electrode 105 is used, a later etchingstep can be easily performed.

After that, a resist mask is formed in a photolithography step over theconductive film 110 and then the conductive film 110 is etched with theuse of the resist mask so that the second electrode 109 is formed (seeFIG. 10B).

In this embodiment, the conductive film to be the second electrode 109is etched using an ammonia hydrogen peroxide solution (a mixed solutionof ammonia, water, and a hydrogen peroxide solution) as an etchant;accordingly, the second electrode 109 is formed.

Note that the etching of the conductive film 110 here may be performedby dry etching, wet etching, or both wet etching and dry etching. Inorder to form the second electrode 109 with a desired shape, the etchingconditions (an etchant, etching time, temperature, and the like) areadjusted as appropriate depending on the material.

Next, as illustrated in FIG. 10C, the gate insulating film 111 is formedover the first electrode 105, the oxide semiconductor film 107, and thesecond electrode 109.

An oxide semiconductor film (an oxide semiconductor film in whichhydrogen concentration is reduced and purity is improved) which is madeintrinsic or substantially intrinsic by removal of an impurity is highlysensitive to an interface state and interface charge; thus, an interfacebetween the oxide semiconductor film and the gate insulating film 111 isimportant. Therefore, the gate insulating film 111 which is in contactwith the highly purified oxide semiconductor film needs high quality.

For example, a high-density plasma CVD apparatus with use of microwaves(2.45 GHz) is preferably employed because formation of a dense andhigh-quality insulating film having high withstand voltage is possible.This is because when the oxide semiconductor film in which hydrogenconcentration is reduced and purity is improved is closely in contactwith the high-quality gate insulating film, the interface state can bereduced and interface properties can be favorable. In addition, sincethe insulating film formed using the high-density plasma CVD apparatuscan have a uniform thickness, the insulating film has excellent stepcoverage. In addition, as for the insulating film formed using thehigh-density plasma CVD apparatus, the thickness of a thin film can becontrolled precisely.

Needless to say, when an insulating film that is favorable as a gateinsulating film can be formed, other film formation methods such as asputtering method and a plasma CVD method can be employed. In addition,any insulating film can be used as long as film quality and interfaceproperties between the oxide semiconductor film and the gate insulatingfilm are modified by heat treatment performed after formation of thegate insulating film. In any case, any insulating film may be used aslong as the insulating film has characteristics of enabling reduction ininterface state density of an interface between the insulating film andthe oxide semiconductor film and formation of a favorable interface aswell as having favorable film quality as a gate insulating film.

In a gate-bias thermal stress test (BT test) at 85° C. and 2×10⁶ V/cmfor 12 hours, if an impurity is added to an oxide semiconductor film,the bond between the impurity and the main component of the oxidesemiconductor film is broken by a high electric field (B: bias) and hightemperature (T: temperature), so that a generated dangling bond inducesa drift in the threshold voltage (V_(th)).

In contrast, the present invention makes it possible to obtain atransistor which is stable to a BT test by removing an impurity in anoxide semiconductor film, especially hydrogen, water, and the like asmuch as possible to obtain a favorable characteristic of an interfacebetween the oxide semiconductor film and a gate insulating film asdescribed above.

Note that when the gate insulating film 111 is formed by sputtering, thehydrogen concentration in the gate insulating film 111 can be reduced.In the case where a silicon oxide film is formed by sputtering, asilicon target or a quartz target is used as a target and oxygen or amixed gas of oxygen and argon is used as a sputtering gas.

Note that a halogen element (e.g. fluorine or chlorine) is contained inan insulating film provided in contact with the oxide semiconductorfilm, or a halogen element is contained in an oxide semiconductor filmby plasma treatment in a gas atmosphere containing a halogen element inthe state where the oxide semiconductor film is exposed, whereby animpurity such as hydrogen, water, hydroxy group, or hydride (alsoreferred to as hydrogen compounds) existing in the oxide semiconductorfilm or at the interface between the oxide semiconductor film and theinsulating film which is provided in contact with the oxidesemiconductor film may be removed. When the insulating film contains ahalogen element, the halogen element concentration in the insulatingfilm may be approximately 5×10¹⁸ cm⁻³ to 1×10²⁰ cm⁻³.

As described above, in the case where a halogen element is contained inthe oxide semiconductor film or at the interface between the oxidesemiconductor film and the insulating film which is in contact with theoxide semiconductor film and where the insulating film which is providedin contact with the oxide semiconductor film is an oxide insulatingfilm, the oxide insulating film on the side where the oxidesemiconductor film is not in contact with the oxide insulating film ispreferably covered with a nitrogen insulating film. That is, a siliconnitride film or the like may be provided on and in contact with theoxide insulating film which is in contact with the oxide semiconductorfilm. With such a structure, an impurity such as hydrogen, water,hydroxy group, or hydride can be prevented from entering the oxideinsulating film.

The gate insulating film 111 can have a structure in which a siliconoxide film and a silicon nitride film are stacked in that order over thefirst electrode 105, the oxide semiconductor film 107, and the secondelectrode 109. For example, a gate insulating film having a totalthickness of 100 nm may be formed in such a manner that a silicon oxidefilm (SiO_(x) (x>0)) with a thickness greater than or equal to 5 nm andless than or equal to 300 nm is formed as a first gate insulating filmand a silicon nitride film (SiN_(y) (y>0)) with a thickness greater thanor equal to 50 nm and less than or equal to 200 nm is stacked over thefirst gate insulating film as a second gate insulating film bysputtering. In this embodiment, a 100-nm-thick silicon oxide film isformed by RF sputtering in the following conditions: the pressure is 0.4Pa; the high-frequency power is 1.5 kW; and an atmosphere containingoxygen and argon (the flow ratio of oxygen to argon is 1:1 (each flowrate is 25 sccm)) is used.

Further, preheat treatment is preferably performed before the gateinsulating film 111 is formed, in order to remove moisture or hydrogenwhich remains on an inner wall of a sputtering apparatus, on a surfaceof the target, or inside a target material. After the preheat treatment,the substrate or the sputtering apparatus is cooled. Then, the gateinsulating film 111 is formed without exposure to the air. Although acertain level of effect can be obtained when introduction and exhaust ofnitrogen are repeated without heating, it is more preferable to performthe treatment with the inside of the film formation chamber heated.

Next, third heat treatment (preferably at a temperature higher than orequal to 200° C. and lower than or equal to 400° C., for example, higherthan or equal to 250° C. and lower than or equal to 350° C.) may beperformed in an inert gas atmosphere or an oxygen gas atmosphere. By theheat treatment, oxygen is supplied to an oxygen deficiency generated bythe first and second heat treatment, so that it is possible to furtherreduce the oxygen deficiency which serves as a donor, satisfy thestoichiometric proportion, and make the oxide semiconductor film 107closer to an i-type oxide semiconductor film or a substantially i-typeoxide semiconductor film. Note that the third heat treatment may beperformed after formation of any of the following: the third electrode113, the insulating film 117, and the wirings 125 and 129. By the heattreatment, it is possible to diffuse hydrogen or moisture contained inthe oxide semiconductor film into the gate insulating film.

Then, the third electrode 113 functioning as a gate electrode is formedover the gate insulating film 111.

The third electrode 113 can be formed in such a manner that a conductivefilm to be the third electrode 113 is formed over the gate insulatingfilm 111 by a sputtering method, a CVD method, or a vacuum evaporationmethod, a resist mask is formed over the conductive film in aphotolithography step, and the conductive film is etched using theresist mask.

Through the above process, the transistor 145 including the oxidesemiconductor film 107 which is highly purified and whose hydrogenconcentration is reduced can be manufactured.

Next, as illustrated in FIG. 11A, after the insulating film 117 isformed over the gate insulating film 111 and the third electrode 113,contact holes 119 and 123 are formed.

The insulating film 117 is formed using an oxide insulating film such asa silicon oxide film, a silicon oxynitride film, an aluminum oxide film,or an aluminum oxynitride film; or a nitride insulating film such as asilicon nitride film, a silicon nitride oxide film, an aluminum nitridefilm, or an aluminum nitride oxide film. Alternatively, an oxideinsulating film and a nitride insulating film can be stacked.

The insulating film 117 is formed by a sputtering method, a CVD method,or the like. Note that when the insulating film 117 is formed by asputtering method, the substrate 101 is heated to a temperature of 100°C. to 400° C., a sputtering gas in which hydrogen, water, hydroxy group,hydride, or the like is removed and which contains high-purity nitrogenis introduced, and an insulating film may be formed using a siliconsemiconductor target. Also in this case, an insulating film ispreferably formed while hydrogen, water, hydroxy group, hydride, or thelike remaining in the treatment chamber is removed.

After the formation of the insulating film 117, heat treatment may befurther performed at a temperature higher than or equal to 100° C. andlower than or equal to 200° C. in the air for greater than or equal to 1hour and less than or equal to 30 hours. With this heat treatment, anormally-off transistor can be obtained. Thus, the reliability of adisplay device or a semiconductor device can be increased.

A resist mask is formed in a photolithography step, and parts of thegate insulating film 111 and the insulating film 117 are removed byselective etching, whereby the contact holes 119 and 123 which reach thefirst electrode 105, the second electrode 109, and the third electrode113 are formed.

Next, after a conductive film is formed over the gate insulating film111 and the contact holes 119 and 123, etching is performed using aresist mask formed in a photolithography step, whereby the wirings 125,129, and 131 are formed (see FIG. 11B). Note that a resist mask may beformed by an ink-jet method. No photomask is used when a resist mask isformed by an ink-jet method; therefore, production cost can be reduced.

The wirings 125, 129, and 131 can be formed in a manner similar to thatof the first electrode 105.

Note that a planarization insulating film for planarization may beprovided between the third electrode 113 and the wirings 125, 129, and131. An organic material having heat resistance, such as polyimide,acrylic, benzocyclobutene, polyamide, or epoxy can be given as typicalexamples of the planarization insulating film. Other than such organicmaterials, it is also possible to use a low-dielectric constant material(a low-k material), a siloxane-based resin, phosphosilicate glass (PSG),borophosphosilicate glass (BPSG), or the like. Note that theplanarization insulating film may be formed by stacking a plurality ofinsulating films formed using these materials.

Note that the siloxane-based resin corresponds to a resin including aSi—O—Si bond formed using a siloxane-based material as a startingmaterial. The siloxane-based resin may include an organic group (e.g.,an alkyl group or an aryl group) or a fluoro group as a substituent.Moreover, the organic group may include a fluoro group.

There is no particular limitation on the method for forming theplanarization insulating film. The planarization insulating film can beformed, depending on the material, by a method such as sputtering, anSOG method, a spin coating method, a dipping method, a spray coatingmethod, or a droplet discharge method (e.g., an ink-jet method, screenprinting, or offset printing), or a tool such as a doctor knife, a rollcoater, a curtain coater, or a knife coater.

As described above, the hydrogen concentration in the oxidesemiconductor film can be reduced, the oxide semiconductor film can behighly purified, and the crystallinity of the oxide semiconductor filmcan be improved. Thus, the oxide semiconductor film can be stabilized.In addition, heat treatment at a temperature of lower than or equal tothe glass transition temperature makes it possible to form an oxidesemiconductor film with a wide band gap in which carrier density isextremely low. Therefore, a transistor can be manufactured using alarge-sized substrate, so that productivity can be increased. Inaddition, by using the oxide semiconductor film in which the hydrogenconcentration is reduced and purity is improved, it is possible tomanufacture a transistor with high withstand voltage, a reducedshort-channel effect, and a high on-off ratio.

Embodiment 6 can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 7

In this embodiment, a method for manufacturing a transistor having astructure different from that of Embodiment 6 will be described withreference to FIGS. 9A to 9E and FIGS. 13A to 13C. This embodiment andEmbodiment 6 are different in a step of forming the conductive film tobe the second electrode 109.

Similarly to Embodiment 6, through the steps illustrated in FIGS. 9A to9D, over the substrate 101, the insulating film 103, the first electrode105, the first oxide semiconductor film 102 b having a polycrystallineregion at least on the surface thereof formed by the first heattreatment, and the second oxide semiconductor film 104 a are formed.

Then, as illustrated in FIG. 13A, the conductive film 110 to be thesecond electrode 109 is formed over the second oxide semiconductor film104 a. Here, the conductive film 110 is formed using a metal elementmelting point of which is higher than or equal to 1000° C. Typicalexamples of a material for the conductive film 110 include molybdenum,tungsten, titanium, tantalum, niobium, iridium, vanadium, chromium,zirconium, platinum, palladium, scandium, iron, yttrium, cobalt, nickel,manganese, and gold.

After that, by performing the second heat treatment, crystal growth iscaused with the use of the polycrystalline region of the first oxidesemiconductor film 102 b as a seed crystal. The second heat treatment isperformed at a temperature higher than or equal to 450° C. and lowerthan or equal to 850° C., preferably higher than or equal to 600° C. andlower than or equal to 700° C. By the second heat treatment, the secondoxide semiconductor film 104 a is crystallized so that the oxidesemiconductor film 108 can be obtained.

After that, a resist mask is formed in a photolithography step over theconductive film 110 and then the conductive film 110 is etched with theuse of the resist mask so that the island-shaped oxide semiconductorfilm 107 and the second electrode 109 are formed (see FIG. 13C).

After that, through the steps described in Embodiment 6 with referenceto FIG. 10C and FIGS. 11A and 11B, the transistor 145 can bemanufactured.

Embodiment 7 can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 8

In this embodiment, a method for manufacturing a transistor having astructure different from those of Embodiments 6 and 7 will be describedwith reference to FIGS. 9A to 9E and FIGS. 14A and 14B. This embodimentand Embodiments 6 and 7 are different in a step of forming the secondoxide semiconductor film.

Similarly to Embodiment 6, through the steps illustrated in FIGS. 9A to9C, over the substrate 101, the insulating film 103, the first electrode105, and the first oxide semiconductor film 102 b having apolycrystalline region at least on the surface thereof formed by thefirst heat treatment are formed as illustrated in FIG. 14A.

After that, as illustrated in FIG. 14B, a second oxide semiconductorfilm 112 is deposited over the first oxide semiconductor film 102 b by asputtering method while heating is performed at a temperature higherthan or equal to 200° C. and lower than or equal to 600° C., preferablyhigher than or equal to 200° C. and lower than or equal to 550° C. Here,the second oxide semiconductor film 112 is deposited while crystalgrowth is caused with the use of the polycrystalline region at thesurface of the first oxide semiconductor film 102 b as a seed crystal sothat the direction of a crystal axis (the c-axis, in particular) of thefirst oxide semiconductor film 102 b and that of the second oxidesemiconductor film 112 are identical (the crystal growth also referredto as epitaxial growth or axial growth). As a result, without the secondheat treatment, the crystallized oxide semiconductor film 108 whose thedirection of c-axis is identical to that of the first oxidesemiconductor film 102 b can be formed. Note that the oxidesemiconductor film 108 includes the first oxide semiconductor film 102 band the second oxide semiconductor film 112.

After that, through the steps described in Embodiment 6 or 7, thetransistor 145 can be manufactured.

In this embodiment, the number of times of heat treatment can bereduced, whereby throughput can be increased.

Embodiment 8 can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 9

In this embodiment, a mode different from Embodiments 6 to 8 isdescribed with reference to FIGS. 15A to 15C.

An example in which the first oxide semiconductor film 102 a has athickness of 15 nm is described in this embodiment.

Although it depends on conditions such as materials of the first oxidesemiconductor film 102 a and the first electrode 105 that is a basemember, heating time, and heating temperature, when the first oxidesemiconductor film 102 a has a thickness of 15 nm, the tips of crystalsof a polycrystalline region 151 does not reach an interface with thefirst electrode 105 and an amorphous region 153 remains, even thoughcrystal growth is caused from the surface of the first oxidesemiconductor film 102 b by the first heat treatment (see FIG. 15A).

Here, FIG. 15B is a cross-sectional view just after deposition of thesecond oxide semiconductor film 104 a over the first oxide semiconductorfilm 102 b.

Then, the second heat treatment is performed after the second oxidesemiconductor film 104 a is formed. By the second heat treatment, in thefirst oxide semiconductor film 102 b, downward crystal growth proceedsto the interface with the first electrode 105; accordingly, the firstoxide semiconductor film 102 b becomes a first oxide semiconductor film102 c in which crystal growth reaches the first electrode 105. In thecase where an oxide semiconductor material of the first oxidesemiconductor film 102 c and that of the second oxide semiconductor film104 b contain the same main components, as illustrated in FIG. 15C,upward crystal growth proceeds to the surface of the second oxidesemiconductor film 104 b with the use of the polycrystalline region ofthe first oxide semiconductor film 102 b as a seed crystal, so that thesecond oxide semiconductor film 104 b is formed and the oxidesemiconductor film 108 having a uniform crystal structure is formed.Therefore, although indicated by a dotted line in FIG. 15C, a boundarybetween the first oxide semiconductor film 102 c and the second oxidesemiconductor film 104 b may become unclear.

In this embodiment, from the interface between the first oxidesemiconductor film 102 b and the second oxide semiconductor film 104 a,crystal growth can proceed both upward and downward.

The conditions of the first and second heat treatment are in the rangewhich is described in Embodiment 6. Note that when the second heattreatment is performed at a temperature higher than the first heattreatment or for a heating time longer than the first heat treatment, asingle crystal region is formed at the surface of the second oxidesemiconductor film 104 a at the time of the second heat treatment insome cases. In the case where such a single crystal region formed at thesurface of the second oxide semiconductor film 104 a affects thetransistor characteristics, conditions which do not allow a singlecrystal region to be formed at the surface of the second oxidesemiconductor film 104 b may be appropriately selected.

In the above description, an example in which the first oxidesemiconductor film has a thickness of 15 nm is described; however, thepresent invention is not particularly limited thereto. Even when thefirst oxide semiconductor film 102 b has a thickness less than or equalto 10 nm, the amorphous region 153 can remain between the single crystalregion 151 of the first oxide semiconductor film 102 b and the firstelectrode 105 by reducing the temperature of the first heat treatment orby shortening the heating time of the first heat treatment. The stateillustrated in FIG. 15C can be realized by forming the single crystalregion reaching the first electrode 105 by the second heat treatment.That is, by employing the process described in this embodiment, thetemperature of the first heat treatment can be reduced and the heatingtime of the first heat treatment can be shortened.

In addition, this embodiment can be arbitrarily combined with otherembodiments.

Embodiment 10

In Embodiments 6 to 9, an oxide semiconductor material of the firstoxide semiconductor film and that of the second oxide semiconductor filmcontain the same main components; this embodiment describes the casewhere a component of an oxide semiconductor material of the first oxidesemiconductor film is different from that of an oxide semiconductormaterial of the second oxide semiconductor film. Note that since FIG.16A is the same as FIG. 9A, the same portions as those in FIG. 9A aredenoted by the same reference numerals in FIG. 16A.

In this embodiment, an example in which a metal oxide target having acomposition ratio of In:Zn=1:1 [atom ratio], which does not contain Gais used and the first oxide semiconductor film has a thickness of 5 nmis described.

FIG. 16A illustrates the first oxide semiconductor film 102 b over theinsulating film 103 and the first electrode 105 after the first heattreatment is performed for crystallization similarly to Embodiment 6.Note that since FIG. 16A is the same as FIG. 9A, the same portions asthose in FIG. 9A are denoted by the same reference numerals in FIG. 16A.

Next, the first heat treatment is performed. Crystal growth proceedsfrom the surface by the first heat treatment, so that the first oxidesemiconductor film 102 b having a polycrystalline region reaching theinterface with the first electrode 105 is formed, which depends onconditions such as materials of the first oxide semiconductor film andthe first electrode 105, heating time, and heating temperature (see FIG.16A).

Crystal growth proceeds from the surface in the perpendicular directionin the first oxide semiconductor film 102 b which has thepolycrystalline region having relatively uniform crystal alignment atthe surface. Further, the first oxide semiconductor film 102 b is c-axisaligned in the direction perpendicular to the surface.

Next, FIG. 16B is a cross-sectional view just after deposition of asecond oxide semiconductor film 161 a over the first oxide semiconductorfilm 102 b. When the second oxide semiconductor film is formed over thefirst oxide semiconductor film, a metal oxide target having acomposition ratio of In:Ga:Zn=1: greater than or equal to 0 and lessthan or equal to 2: greater than or equal to 1 and less than or equal to5 is used. In this embodiment, as the second oxide semiconductor film161 a, an In—Ga—Zn—O film is formed to a thickness of 1 μm using anIn—Ga—Zn—O-based oxide semiconductor target (In:Ga:Zn=1:1:1 [atomratio]).

Then, the second heat treatment is performed after the second oxidesemiconductor film 161 a is formed. By the second heat treatment,crystal growth is caused as illustrated in FIG. 16C. As illustrated inFIG. 16C, upward crystal growth proceeds to the surface of the secondoxide semiconductor film with the use of the polycrystalline region ofthe first oxide semiconductor film 102 b as a seed crystal; accordingly,a second oxide semiconductor film 161 b can be formed.

The first oxide semiconductor film 102 b becomes the third oxidesemiconductor film 102 c whose crystallinity is further improved becausethe polycrystalline region obtained by the first heat treatment isheated again by the second heat treatment.

Since the component of the oxide semiconductor material of the secondoxide semiconductor film 161 a is different from that of the oxidesemiconductor material of the first oxide semiconductor film 102 b, aboundary is formed between the third oxide semiconductor film 102 c andthe second oxide semiconductor film 161 b as illustrated in FIG. 16C.Also by the second heat treatment, most part of the first oxidesemiconductor film, including part near the interface with the firstelectrode 105, becomes the polycrystalline region.

The structure illustrated in FIG. 16C can be referred to as a two-layerstructure in which the third oxide semiconductor film 102 c and thesecond oxide semiconductor film 161 b are stacked in this order over andin contact with the first electrode 105. Upward crystal growtheffectively proceeds to make the In—Ga—Zn—O film be a polycrystallineregion by using the In—Zn—O film, which is crystallized more easily thanan In—Ga—Zn—O film, as the seed crystal. The band gap of the third oxidesemiconductor film 102 c can be different from that of the second oxidesemiconductor film 161 b.

The conditions of the first and second heat treatment are in the rangewhich is described in Embodiment 6.

In addition, this embodiment can be arbitrarily combined with otherembodiments.

Embodiment 11

In this embodiment, a manufacturing method of a semiconductor devicewith high yield is described with reference to FIGS. 17A and 17B.

As illustrated in FIG. 17A, the insulating film 103 is formed over thesubstrate 101, and the first electrode 105 is formed over the insulatingfilm 103. Then, a protective film 165 is formed over the first electrode105. The protective film 165 is provided in order to increase adhesionof the first electrode 105 to an oxide semiconductor film to be formedlater. Further, the protective film 165 is provided in order to preventthe first electrode 105 to be oxidized in a step of forming the oxidesemiconductor film.

A metal nitride film having a thickness greater than or equal to 1 nmand less than or equal to 100 nm is preferably formed as the protectivefilm 165; typically, a titanium nitride film or a tantalum nitride filmis formed.

The first oxide semiconductor film 102 a is then formed, whereby peelingof the first oxide semiconductor film 102 a can be suppressed. Inaddition, the first electrode 105 can be prevented from being oxidized.

Then, the steps which are described in any of Embodiments 6 to 10 areperformed; thus, a semiconductor device can be manufactured.

Embodiment 12

A mode which uses a circuit which includes the semiconductor elementdescribed in any of Embodiments 1 to 11 will be described.

A transistor and a diode, each of which is one mode of the semiconductorelement described in any of Embodiments 1 to 11, have a high on-offratio and high withstand voltage and is scarcely degraded. Thus, thetransistor and diode can be used in the following examples: a homeelectrical appliance in which an inverter technique is applied such asan air conditioner, a refrigerator, a rice cooker, or a solar powergeneration system; a battery-driven portable information terminal devicesuch as a laptop computer; a power amplifier device such as astroboscope, an electric vehicle; a DC-DC converter circuit; a motorcontrol circuit; an audio amplifier; a logic circuit; a switch circuit;and a high-frequency linear amplifier.

Here, an example of a solar power generation system which is providedwith an inverter formed using the semiconductor element described in anyof Embodiments 1 to 11 is described with reference to FIG. 18. Note thatan example of a structure of a solar power generation system installedon a house and the like is described here.

A residential solar power generation system illustrated in FIG. 18 is asystem in which a method for supplying electric power is changed inaccordance with the state of solar power generation. When solar powergeneration is performed, for example, when the sun shines, electricpower generated by solar power generation is consumed inside the house,and surplus electric power is supplied to an electric grid 414 providedby an electric power company. On the other hand, at night time or at thetime of rain when electric power is insufficient, electric power issupplied from the electric grid 414 and is consumed inside the house.

The residential solar power generation system illustrated in FIG. 18includes a solar cell panel 400 which converts sunlight into electricpower (direct current power), an inverter 404 which converts theelectric power from direct current into alternating current, and thelike. Alternating current power output from the inverter 404 is used aselectric power for operating various types of electric devices 410.

Surplus electric power is supplied to outside the house through theelectric grid 414. That is, electric power can be sold using thissystem. A direct current switch 402 is provided to select connection ordisconnection between the solar cell panel 400 and the inverter 404. Analternating current switch 408 is provided to select connection ordisconnection between a distribution board 406 and a transformer 412connected to the electric grid 414.

When the semiconductor device of the disclosed invention is applied tothe above inverter, a highly reliable and inexpensive solar powergeneration system can be realized.

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

EXAMPLE 1

In Example 1, results of taking TEM photographs of a cross section ofoxide semiconductor films which were crystallized by heat treatment aredescribed with reference to FIGS. 19A and 19B and FIGS. 20A and 20B.

First, a manufacturing method of Sample A is described below.

A silicon oxynitride film (SiON) was formed over a glass substrate by aCVD method. Then, an In—Ga—Zn—O film (OS) having a thickness of 5 nm wasformed over the silicon oxynitride film. At this time, the In—Ga—Zn—Ofilm was formed in the following conditions: an oxide semiconductortarget (an In—Ga—Zn—O-based oxide semiconductor target(In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio] and In:Ga:Zn=1:1:1 [atom ratio])was used; the substrate temperature was 200° C.; the deposition rate was4 nm/min; and the target was sputtered. Note that in the case where theabove-described oxide semiconductor target is used, a crystal ofInGaZnO₄ is easily obtained. Next, a protective film was formed over theIn—Ga—Zn—O film. The In—Ga—Zn—O film formed over the glass substrate wasthen subjected to heat treatment in a dry air atmosphere at 700° C. for1 hour; thus, Sample A was manufactured.

FIG. 19A is a TEM photograph of a cross section of Sample A and FIG. 19Bis a schematic diagram of FIG. 19A. Note that the TEM photograph is ahigh magnification photograph (eight million-fold magnification) takenat an acceleration voltage of 300 kV using a high resolutiontransmission electron microscope (“H9000-NAR”: TEM manufactured byHitachi, Ltd.). It can be observed that the In—Ga—Zn—O film was c-axisaligned in a direction perpendicular to the surface and a region closeto the interface between the silicon oxynitride film and the In—Ga—Zn—Ofilm was also crystallized and c-axis aligned in a directionperpendicular to the surface. In other words, an oxide semiconductorfilm having a flat-plate-shaped polycrystalline region was formed. Notethat elements which were adjacent to each other in the a-b plane were ofthe same kind. The c-axis direction of the flat-plate-shapedpolycrystalline region corresponded to the direction perpendicular tothe surface.

Next, a manufacturing method of Sample B, which is a comparativeexample, is described below.

A silicon oxynitride film (SiON) was formed over a glass substrate by aCVD method. Then, an In—Ga—Zn—O film having a thickness of 50 nm wasformed over the silicon oxynitride film in conditions similar to thoseof Sample A. Next, a protective film was formed over the In—Ga—Zn—Ofilm. After that, heat treatment was performed in a dry air atmosphereat 700° C. for 1 hour; thus, Sample B was manufactured.

FIG. 20A is a TEM photograph of a cross section of Sample B and FIG. 20Bis a schematic diagram of FIG. 20A. Note that the TEM photograph is ahigh magnification photograph (two million-fold magnification) taken atan acceleration voltage of 300 kV using a high resolution transmissionelectron microscope (“H9000-NAR”: TEM manufactured by Hitachi, Ltd.). Itcan be observed that crystallization proceeded to a depth ofapproximately 5 nm from the surface of the

In—Ga—Zn—O film and a lot of amorphous regions and a plurality ofcrystals whose crystal axes were not uniformly oriented randomly existedin the inside portion of the In—Ga—Zn—O film. Therefore, it can be saidthat even when heat treatment at 700° C. for 1 hour, i.e., heattreatment at a temperature higher than 650° C. for a treatment timelonger than 6 minutes, is performed once, a single crystal region havinghigh alignment is hardly formed in a whole In—Ga—Zn—O film which isformed as thick as 50 nm

From these experimental results, it can be said that a polycrystallineregion having a large thickness can be formed by forming an oxidesemiconductor film by the following two steps: a polycrystalline regionto be a seed crystal is formed; and then crystal growth is caused afteranother oxide semiconductor film is formed. That is, it is shown thatthe method disclosed in this specification is extremely effective.Formation of an oxide semiconductor film in two steps and heat treatmentperformed twice make it possible to obtain a thick polycrystallineregion having high alignment, i.e., a polycrystalline region having thea-b plane which is parallel to the surface of the flat-plate-shapedpolycrystalline region and being c-axis aligned in a directionperpendicular to the surface of the flat-plate-shaped polycrystallineregion.

This application is based on Japanese Patent Application serial no.2009-270854 filed with Japan Patent Office on Nov. 28, 2009, the entirecontents of which are hereby incorporated by reference.

1. An oxide semiconductor film on an amorphous insulating surface, theoxide semiconductor film comprising indium, zinc, a metal element otherthan indium and zinc, and oxygen, wherein the oxide semiconductor filmis highly purified, wherein the oxide semiconductor film is intrinsic orsubstantially intrinsic, and wherein the oxide semiconductor filmincludes a plurality of crystalline regions, c-axes of which aresubstantially perpendicular to a surface of the oxide semiconductorfilm.
 2. The oxide semiconductor film according to claim 1, wherein themetal element is gallium.
 3. The oxide semiconductor film according toclaim 1, wherein the oxide semiconductor film is provided on a siliconoxynitride, and wherein the silicon oxynitride includes the amorphousinsulating surface.
 4. An oxide semiconductor film on an amorphousinsulating surface, the oxide semiconductor film comprising indium,zinc, a metal element other than indium and zinc, and oxygen, whereinthe oxide semiconductor film includes a plurality of crystallineregions, c-axes of which are substantially perpendicular to a surface ofthe oxide semiconductor film, and wherein a concentration of iron in theoxide semiconductor film is 1.0×10¹⁵ atoms/cm³ or lower.
 5. The oxidesemiconductor film according to claim 4, wherein the metal element isgallium.
 6. The oxide semiconductor film according to claim 4, whereinthe oxide semiconductor film is provided on a silicon oxynitride, andwherein the silicon oxynitride includes the amorphous insulatingsurface.
 7. An oxide semiconductor film on an amorphous insulatingsurface, the oxide semiconductor film comprising indium, zinc, a metalelement other than indium and zinc, and oxygen, wherein the oxidesemiconductor film includes a plurality of crystalline regions, c-axesof which are substantially perpendicular to a surface of the oxidesemiconductor film, and wherein a concentration of nickel in the oxidesemiconductor film is 1.0×10¹⁵ atoms/cm³ or lower.
 8. The oxidesemiconductor film according to claim 7, wherein the metal element isgallium.
 9. The oxide semiconductor film according to claim 7, whereinthe oxide semiconductor film is provided on a silicon oxynitride, andwherein the silicon oxynitride includes the amorphous insulatingsurface.
 10. An oxide semiconductor film on an amorphous insulatingsurface, the oxide semiconductor film comprising indium, zinc, a metalelement other than indium and zinc, and oxygen, wherein the oxidesemiconductor film includes a plurality of crystalline regions, c-axesof which are substantially perpendicular to a surface of the oxidesemiconductor film, and wherein a concentration of hydrogen in the oxidesemiconductor film is 1.0×10¹⁸ atoms/cm³ or lower.
 11. The oxidesemiconductor film according to claim 10, wherein the metal element isgallium.
 12. The oxide semiconductor film according to claim 10, whereinthe oxide semiconductor film is provided on a silicon oxynitride, andwherein the silicon oxynitride includes the amorphous insulatingsurface.
 13. An oxide semiconductor film on an amorphous insulatingsurface, the oxide semiconductor film comprising indium, zinc, a metalelement other than indium and zinc, and oxygen, wherein the oxidesemiconductor film includes a plurality of crystalline regions, c-axesof which are substantially perpendicular to a surface of the oxidesemiconductor film, and wherein a concentration of an impurity impartingan n-type conductivity type to the oxide semiconductor film is reducedso that the oxide semiconductor film is intrinsic or substantiallyintrinsic.
 14. The oxide semiconductor film according to claim 13,wherein the metal element is gallium.
 15. The oxide semiconductor filmaccording to claim 13, wherein the oxide semiconductor film is providedon a silicon oxynitride, and wherein the silicon oxynitride includes theamorphous insulating surface.
 16. An oxide semiconductor film on anamorphous insulating surface, the oxide semiconductor film comprisingindium, zinc, a metal element other than indium and zinc, and oxygen,wherein the oxide semiconductor film is highly purified, wherein theoxide semiconductor film includes a plurality of crystalline regions,c-axes of which are substantially perpendicular to a surface of theoxide semiconductor film, and wherein a density of state caused due tooxygen defects in the oxide semiconductor film is reduced so that theoxide semiconductor film is intrinsic or substantially intrinsic. 17.The oxide semiconductor film according to claim 16, wherein the metalelement is gallium.
 18. The oxide semiconductor film according to claim16, wherein the oxide semiconductor film is provided on a siliconoxynitride, and wherein the silicon oxynitride includes the amorphousinsulating surface.
 19. An oxide semiconductor film on an amorphousinsulating surface, the oxide semiconductor film comprising indium,zinc, a metal element other than indium and zinc, and oxygen, whereinthe oxide semiconductor film comprises a first region and a secondregion over the first region, wherein the first region comprises anamorphous portion and a plurality of crystals, and wherein the secondregion comprises a plurality of crystalline regions, c-axes of which aresubstantially perpendicular to a surface of the oxide semiconductorfilm.
 20. The oxide semiconductor film according to claim 19, whereinthe metal element is gallium.
 21. The oxide semiconductor film accordingto claim 19, wherein the oxide semiconductor film is provided on asilicon oxynitride, and wherein the silicon oxynitride includes theamorphous insulating surface.